add blogger-separate class

This PR adds a blogger converter class which attempts to isolate the notebook formatting from the rest of the page.  This is useful for inserting notebooks into blog posts, without resulting in modifying the CSS style of the entire blog.  For an application, see my pelican-plugins PR: https://github.com/getpelican/pelican-plugins/pull/21

I also used an early version of this approach in my guide to ipynb blogging in octopress: http://jakevdp.github.io/blog/2012/10/04/blogging-with-ipython/

It's a bit hack-ish, using regular expressions and `replace` to modify the CSS.  If this type of output is useful to people (I know it's been extremely useful to me) it may be worth eventually finding a cleaner solution.