Post sort option (settings), markitup & bbcode updates

Hi,
in this PR i'm proposing a few changes, introduced since i'm using pybbm on a project that will be released soon (this weekend, i suppose):

* Added a setting, to support Postview reverse ordering, based on the "created" datefield [PYBB_POST_SORT_REVERSE]
* Updated the markitup section, to latest version (css, js, img)
* Added font color selector to markitup
* Added two new bbcode parsers:
     * Font Size
     * Youtube video embedding

Let me know, if you are interested in this PR, and if you are going to release (sooner or later) a new version to pypi, since i need to decide whether, or not, to  setup a specific buildpack for my heroku deployment.

Thanks