Build and use a lookup table for Ruby-compatible encoding names

CharlockHolmes::EncodingDetector can now potentially return another
key `:ruby_encoding` which is a Ruby-compatible encoding name which
maps to a corresponding encoding name from ICU's encoding detection
API.

For encoding names where no match could be found, `:ruby_encoding`
is set to "binary".
Make the detect_encoding string helper methods use :ruby_encoding
Merge pull request #68 from brianmario/compatible-encoding-table

Add and use a Ruby-compatible encoding lookup table