fixes #2247 Provides an option to disable the use of gravatara

A new setting is added, use_gravatar. It defaults to true. If it is set to false, then the gravatar
call out is not used and only the default user icon is used.
Configurable Katello SSO support

You can use SSO service that comes with Katello for loggin in. It's
based on OpenID protocol with slightly customized provider.

SSO abstractioning

Apache and Signo SSO reworked to a new general SSO concept.

Added test for new SSO concept

Moved rack-openid to separate bundler file

Use FQDN instead of domain for Signo default URL
Code reviews

* changed Class Sso to module SSO
* extracted apache remote user to a variable
* simplied sso classes using delegate
* changed refute to assert ! to work on ruby 1.8.7
* minor text formatting changes