Replace 'webvtt_byte' with 'char' for strings.
Remove unnecessary casting in webvttxx/string
Removed unnecessary casts from cue_testfixture
Merge pull request #361 from caitp/byte-to-char

Replace 'webvtt_byte' with 'char'
Replace 'webvtt_byte' with 'char' in README.md
Merge pull request #362 from caitp/readme-fup

Replace 'webvtt_byte' with 'char' in README.md
Fix bug in webvtt_class_state() func.

Only add buffer to the classes stirng list if we have parsed
one or more characters into the buffer. Not doing this will
result in situations where we have an empty class string in
the string list.
Add class state tokenizer tests.