Added support for webkit for heading formatting

Heading formatting was using the 'FormatBlock' command for msie and
opera only but webkit in my test does not support the 'heading'
alternative. Ternary operator on h1 h2 and h3 buttons was changed to
only use 'heading' for Firefox and 'FormatBlock' for all other browsers.
Merge pull request #373 from bensventures/master

Added support for webkit for heading formatting