Allow custom label to be set for authentication providers.

Addresses internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2341

Beside the above, I've:

• Refactored OauthHelper to have clearer method names and behaviour
• Moved some of OauthHelper behaviour to Gitlab::OAuth::Provider
• Renamed OauthHelper to AuthHelper since LDAP, SAML, Kerberos aren't OAuth
• Updated the icons for GitHub and GitLab

In the examples below, "OurAuth" is a SAML provider with a custom label.