Fixes #282 - Added 'default' cases in parser.c where needed.

I've refactored webvtt_parse_align very slightly, so that it no longer
needs branching or switch statement at all.

Other switch statements now have 'default' branches, which generally
don't do anything. Some of them probably need to, however. Better test
coverage should reveal problems with them.
Merge pull request #282 from caitp/issue-282

[warning] enumeration value XXXX not handled in switch (parser.c)