Align td and th with css instead of align property

When you align tables with css, \<td align="..."\> gets overridden. 
Using inline styles keeps alignment no matter what CSS contains.

While this is a departure from what Github currently does, the reference markdown parser they use, [RedCarpet](https://github.com/vmg/redcarpet) has made [this change](https://github.com/vmg/redcarpet/commit/1e61096be1c5e7cd34d19ca65afa2fca87639c5f), and I'm hoping Github will follow at some point in the future.