Add Mac and Windows installer launching

Also starts a detached process so the window doesn't get held open
(many GUI apps launch the interface in a new process, so xdg-open
returns immediately, but it's not guaranteed).

app/main.js

@@ -16,6 +16,29 @@ let subpy
 // set to true when the quitting sequence has started
 let quitting = false;
 
+/*
+ * Replacement for Electron's shell.openItem. The Electron version doesn't
+ * reliably work from the main process, and we need to be able to run it
+ * when no windows are open.
+ */
+function openItem(fullPath) {
+    const subprocOptions = {
+      detached: true,
+      stdio: 'ignore',
+    };
+
+    let child;
+    if (process.platform == 'darwin') {
+      child = child_process.spawn('open', [fullPath], subprocOptions);
+    } else if (process.platform == 'linux') {
+      child = child_process.spawn('xdg-open', [fullPath], subprocOptions);
+    } else if (process.platform == 'win32') {
+      child = child_process.execSync('start', [fullPath], subprocOptions);
+    }
+
+    // Causes child process reference to be garbage collected, allowing main process to exit
+    child.unref();
+}
 
 function createWindow () {
   win = new BrowserWindow({backgroundColor: '#155b4a'})
@@ -177,11 +200,9 @@ function shutdown() {
 
 function upgrade(event, installerPath) {
   app.on('quit', () => {
-    // shell.openItem doesn't reliably work from the app process, so run xdg-open directly
+    openItem(installerPath);
-    child_process.spawn('xdg-open', [installerPath], {
-      stdio: 'ignore',
-    });
   });
+
   if (win) {
     win.loadURL(`file://${__dirname}/dist/upgrade.html`);
   }

ui/js/app.js

@@ -24,7 +24,6 @@ import {Link} from './component/link.js';
 
 const {remote, ipcRenderer, shell} = require('electron');
 const {download} = remote.require('electron-dl');
-const os = require('os');
 const path = require('path');
 const app = require('electron').remote.app;
 const fs = remote.require('fs');
@@ -44,23 +43,29 @@ var App = React.createClass({
   _version: null,
 
   getUpdateUrl: function() {
-    switch (os.platform()) {
+    switch (process.platform) {
       case 'darwin':
         return 'https://lbry.io/get/lbry.dmg';
       case 'linux':
         return 'https://lbry.io/get/lbry.deb';
       case 'win32':
-        return 'https://lbry.io/get/lbry.exe'; // should now be msi
+        return 'https://lbry.io/get/lbry.exe';
+      default:
+        throw 'Unknown platform';
     }
   },
-  // Temporary workaround since electron-dl throws errors when you try to get the filename
+  // Hard code the filenames as a temporary workaround, because
+  // electron-dl throws errors when you try to get the filename
   getUpgradeFilename: function() {
-    if (os.platform() == 'darwin') {
+    switch (process.platform) {
-      return `LBRY-${this._version}.dmg`;
+      case 'darwin':
-    } else if (os.platform() == 'linux') {
+        return `LBRY-${this._version}.dmg`;
-      return `LBRY_${this._version}_amd64.deb`;
+      case 'linux':
-    } else {
+        return `LBRY_${this._version}_amd64.deb`;
-      return `LBRY.Setup.${this._version}.exe`;
+      case 'windows':
+        return `LBRY.Setup.${this._version}.exe`;
+      default:
+        throw 'Unknown platform';
     }
   },
   getInitialState: function() {