one gemset per .ruby-version file

i wanted to have separate gemsets for all of my projects
i modified auto.sh to export the location of the .ruby-version file
and changed chruby.sh to set GEM_PATH and GEM_HOME to use that location
this way i do not have to use chgems at all, all i have to do is make a .ruby-version file in the root of my project (and a Gemfile, and install it with gem install -g Gemfile)

this pull request can not be merged as is
i want your opinion first, if you agree that it is a worthy addition, i can make it mergable

i like the idea to have one program to do one thing only
but this hack was too easy not to do it