Added PKCS12 support for user-cred, fixes #48

* Works on Ruby 1.8.7 - 1.9.3, tested
* Doesn't work on jRuby since it doesn't have a full
  implementation of OpenSSL::PKCS12
* Detects PKCS12 credentials automatically based on
  the file suffix 'p12' (e.g., ~/.globus/usercred.p12)