A few minor modifications to new base58 package.

- Call out in README.md that this is modified base58 (it's not the same as
  normal base58)
- Remove the blurb about test_coverage.txt since it is no longer needed
  now that the repo now has coveralls integrated
- Rename base58_check[_test].go -> basecheck[_test].go.  Since Go treats
  _<ext> special in some cases like for tests and conditional OS and
  architecture compilation, it's a good idea to avoid naming files with
  them to ensure a new special meaning doesn't break builds in the future

base58/README.md

@@ -4,13 +4,12 @@ base58
 [![Build Status](https://travis-ci.org/conformal/btcutil.png?branch=master)]
 (https://travis-ci.org/conformal/btcutil)
 
-Package base58 provides an API for for encoding and decoding to and from the base58 encoding.
-It also provides an API to do base58Check encoding, as described [here](https://en.bitcoin.it/wiki/Base58Check_encoding). 
+Package base58 provides an API for for encoding and decoding to and from the
+modified base58 encoding.  It also provides an API to do base58Check encoding,
+as described [here](https://en.bitcoin.it/wiki/Base58Check_encoding).
 
-A comprehensive suite of tests is provided to ensure proper functionality.  See
-`test_coverage.txt` for the gocov coverage report.  Alternatively, if you are
-running a POSIX OS, you can run the `cov_report.sh` script for a real-time
-report.  Package base58 is licensed under the liberal ISC license.
+A comprehensive suite of tests is provided to ensure proper functionality.
+Package base58 is licensed under the copyfree ISC license.
 
 ## Documentation