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