Added th to table headers so that styling with things like Twitter Bootstrap and typeset.css work as expected. Cells in headers should always be TH unless they are advisory cells within headers in which case TD is acceptable (but being Markdown a user with such needs could just enter HTML for this)
Updated tests to check for th tags instead of td tags within thead blocks
Merge pull request #43 from microcosm-cc/master

Cells in THEAD > TR are now TH.