Split rvm/rvm_scripts to allow for other version management

rvm_scripts stays an alias of env_scripts to keep current configs
working
this allows putting something like this:
env_scripts '/etc/profile.d/rbenv.sh'
in your config and have rbenv work(assuming you setup that sh)
Update readme to include env_scripts setup
Merge pull request #75 from danielfarrell/rbenv

Split rvm/rvm_scripts to allow for other version management