Remove `text-align:left` from github.css

GFM has support for specifying alignment in tables by using colons in the header row (see https://help.github.com/articles/github-flavored-markdown#tables). Currently github.css overrides this by forcing left alignment for no good reason.
Merge pull request #322 from tiziano88/patch-1

Remove `text-align:left` from github.css