Use ert-runner for running tests.
Add function rvm-activate-ruby-for.
Add Makefile to run tests.
Protect against errors in callback.

If an error occurs in the callback to rvm-activate-ruby-for, we still
want to reset to the old state.
Improve Travis setup.
Merge pull request #39 from rejeep/master

Add rvm-activate-ruby-for
Mention rvm-activate-ruby-for in usage section.
Update instructions for contribution.
Merge pull request #40 from rejeep/readme

Readme