Add .gitignore file

For the convenience of dev and testing, I set up a special gemset for
the repo with RVM. It uses Ruby 2.0.0. I added a .ruby-version file so I
don't have to ask RVM to change gemset for me each time I enter the
folder. Then I put in in .gitignore to avoid interfering with other
people's setup.
Add a line in .gitignore

Also ignore .ruby-gemset on the basis of previous commit.