Support user-defined Tables

This pull request adds minimal code that supports users specifying their
own table to use. This case is useful if you prefer not to have vowels
or a different set of characters entirely.

```ruby
require 'base32'

Base32.table = "1bcd2fgh3jklmn4pqrstAvwxyz567890".freeze
encoded = Base32.encode("chunky bacon!")  #==> "mnAhk6tlp2qg2yldn8xcc==="
decoded = Base32.decode(encoded)          #==> "chunky bacon!"
```

I also had to fix the rake test files glob, it was not working with the
name of the test file.