Fixes #303 - Review PayloadFormat.MultilineMultipleCueTextTag test

- Assert node childCount to prevent segv
- Don't expect head-node to have kind other than Head
- EXPECT rather than ASSERT when failure is non-fatal
- More meaningful variable names (italicNode vs node1, etc)
- Wrap lines to 80columns
Removed plformat_unittest from XFAIL_TESTS
Merge branch 'issue-303' of git://github.com/caitp/webvtt into issue-303-rebase

Conflicts:
	test/unit/plformat_unittest.cpp
Merge pull request #314 from RickEyre/issue-303-merge

Issue 303 merge