Refine CLA Structure

This removes some unneeded columns from User. It also removes the ability
to update CLAs. Instead users can re-sign previously signed CLAs from
the show actions. The difference being users can update their contact information
if they desire but the action of re-signing creates a new signature therefore keeping the
full history of signatures made by the user or company for any legal obligations.
Merge pull request #96 from opscode/cla-improvements

Refine CLA Structure