Require only RVM environment not full path, and use rvm use.

Sourcing the RVM environment using the full path only works if that
environment has been configured to source from a script. Load the RVM
environment requiring only `ruby-2.0.0-p0@tunefl`-style syntax, changing
to use `rvm use` within helper scripts.
Document RVM environment parameters change, simplififying example.
Merge branch 'better-rvm' into develop