Add support for automatically sourcing the right rvm env

tpope/bundler won't work without this plugin on a project
that uses dvm when mvim is not launched from within the
project's directory.
Using the command :Rvm will source the right GEM_HOME
so that :Bedit [gem] is able to look in the right place.
Taking it one step further, :Rvm is issued on BufEnter *
Fixes #267