Rubygems 2.0+ integration

Rubygems 2.0 got rid of DocManager and instead relies on pos-tinstall hook. 

This is a port of the old integration (with RDoc integration code as a basis).

There are a few caveats to this code.

Both YARD and yri data is generated into docs directories supplied by the rubygems. Currently, yri can not find generated docs there. I intend to fix it too.