Fix token and node memory management.

Fixes issue #87. @caitp please review.

- Gone through and started using webvtt_copy_string where applicable.
- Changed cue_text_tokenizer function to initialize it's variables after
checking for invalid params, as well as properly releasing data at the end
of the function
- Changed webvtt_parse_cuetext to properly release the nodes that it creates,
delete the token that it finished with, as well as a small logic error where
it would continue without skipping white space.