Redcarpet Socialable Mixins

I found `redcarpet-socialable` great for its purpose, but unfortunately all I needed is the mentions support.
So I decided to split the codebase into modules/mixins we could use separately.

A couple more changes:
* Updated gemspec dependencies, loosed a couple of libraries. Set up rspec properly.
* Updated some method names to make the API a bit more consistent
* Updated the base regexp to include HTML tags if any so we can properly skip those without MD5 temporary replacements

Although changes seem big. I almost didn't touch the old tests so everything should be 100% backwards compatible.

There are a couple of things I would like to see next, hopefully in nearest future:
* Extended tests for different markdown use cases
* Currently link tags are being skipped (it would be wrong to put a link tag into another), but this is pretty easy to handle
* Updated documentation (let me know if you are a fan of RDoc or Yardoc, or something else)
* CI setup, proabably Travis

Thanks for your work so far, it was a great starting point.
Please take a look and let me know if there's something wrong or needs fixing in order to get this merged.