Add gem security policy option to install command

  * add a --policy option that accepts appropriate values for the 'gem -P' command
  * pass the security policy down to the gem module for install and update events
get rid of eval
Fix uncaught exception

 * this was for the case of a signed gem that does not have appropriate CAs
 * gem code is raising classless exceptions in this and a few other cases. Ugh
Add tests for gem security options upon install/update.

 * Note that signed certs with a CA are untested. Hopefully they are
   tested appropriately at the rubygems level.
 * update is tested via config entry
Merge pull request #2293 from CosmicCat/gem-security-policy

Add gem security policy option to install command
alphabetize-ish and fix require locations
remove many methods
normalize error now that we catch anything
rename policy to trust-policy like gem, docs