Add Generate support in mining.go.

- Create FutureGenerateResult type with Receive() method

- Create GenerateAsync method for Client which returns a
  FutureGenerateResult

- Create Generate method for Client which calls GenerateAsync
  and then calls Receive() on the returned FutureGenerateResult

mining.go

@@ -10,9 +10,56 @@ import (
 	"errors"
 
 	"github.com/btcsuite/btcd/btcjson"
+	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btcutil"
 )
 
+// FutureGenerateResult is a future promise to deliver the result of a
+// GenerateAsync RPC invocation (or an applicable error).
+type FutureGenerateResult chan *response
+
+// Receive waits for the response promised by the future and returns a list of
+// block hashes generated by the call.
+func (r FutureGenerateResult) Receive() ([]*wire.ShaHash, error) {
+	res, err := receiveFuture(r)
+	if err != nil {
+		return nil, err
+	}
+
+	// Unmarshal result as a list of strings.
+	var result []string
+	err = json.Unmarshal(res, &result)
+	if err != nil {
+		return nil, err
+	}
+
+	// Convert each block hash to a wire.ShaHash and store a pointer to each.
+	convertedResult := make([]*wire.ShaHash, len(result))
+	for i, hashString := range result {
+		convertedResult[i], err = wire.NewShaHashFromStr(hashString)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return convertedResult, nil
+}
+
+// GenerateAsync returns an instance of a type that can be used to get
+// the result of the RPC at some future time by invoking the Receive function on
+// the returned instance.
+//
+// See Generate for the blocking version and more details.
+func (c *Client) GenerateAsync(numBlocks uint32) FutureGenerateResult {
+	cmd := btcjson.NewGenerateCmd(numBlocks)
+	return c.sendCmd(cmd)
+}
+
+// Generate generates numBlocks blocks and returns their hashes.
+func (c *Client) Generate(numBlocks uint32) ([]*wire.ShaHash, error) {
+	return c.GenerateAsync(numBlocks).Receive()
+}
+
 // FutureGetGenerateResult is a future promise to deliver the result of a
 // GetGenerateAsync RPC invocation (or an applicable error).
 type FutureGetGenerateResult chan *response