Merge pull request #358 from RickEyre/tokenizer-tests-2

More cue text tokenizer unit tests.
Add cStr() funct to string binding.

There are times when it would be nice to return a const
char * directly from the string binding instead of returning
a const byte * and then having to cast.
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.