Passing all but two 'line' cuesetting tests.

I've integrated the webvtt_parse_line() routine from my large patch,
and am calling it in the same fashion as I am calling the parse_align
routine, so we get some uniform results from the two tests.

There are still two tests failing, which stems from not having a
standard on which errors to return and which errors to expect. These
two other tests will also require unit test changes, and will be
corrected in a followup pull request.
Align/Line now passing 100%

I've also added some convenience functions to the cue testfixture for
asserting that a cue has default cuesettings, just in case the
defaults in the spec change. That way we can address them in that one
location rather than in each individual test.
Merge pull request #267 from caitp/settings-line

Passing 'line' cuesettings tests 100%