Fix for maestrodev issue #20 - rvmrc / rvm group

This is a fix for: https://github.com/maestrodev/puppet-rvm/issues/20

The following changes were made:
 - Added $group parameter, defaulted to 'rvm'
 - Copied Line 8 from system_user.pp to ensure rvm group is created.
Test that compilation succeeds
Use params pattern to avoid repeating rvm group
Create rvm::group to avoid duplication