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.
Rename funcs to use camel case.
Fix memory leak in webvtt_class_state().
Merge pull request #360 from RickEyre/class-state-tokenizer-tests

Class state tokenizer tests