Add chruby_exec and chruby_gem LWRPs

[johnbellone]

This commit adds the LWRP for running chruby-exec and gem commands in the context of a supplied Ruby version.

Additional changes:

• Default to the latest, stable version of chruby;
• Default to the latest, stable version of Ruby 1.9.3;
• Install default gems using LWRP for all rubies;
• Attribute for specifying installation prefix.

[johnbellone]

I'll make sure this jives with #5. I can easily add a chruby_gem LWRP as well.