Moved typedefs to enum definition in ct_internal.h

I moved the typedefs from the top of the code to where the actual enums
are defined in cuetext_internal.h as there was a problem where they
were not visible when including cuetext.c in the new unit tests.
Change cuetext parser to use const webvtt_byte *.
Added unit tests for internal cue text functions.

- Added cuetexttokenizer_testfixture which.
- Added unit tests for the data, escape, and tag tokenizer
functions.
Merge pull request #351 from RickEyre/tokenizer-tests

Tokenizer tests