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.

![Screen_Shot_2015-07-02_at_16.29.52](https://gitlab.com/gitlab-org/gitlab-ce/uploads/7d425bde69dc34e1667ebd5375d0266d/Screen_Shot_2015-07-02_at_16.29.52.png)

![Screen_Shot_2015-07-02_at_16.31.40](https://gitlab.com/gitlab-org/gitlab-ce/uploads/cbb273321ecdf4cab3d3ef0dc35553e7/Screen_Shot_2015-07-02_at_16.31.40.png)

![Screen_Shot_2015-07-02_at_16.32.39](https://gitlab.com/gitlab-org/gitlab-ce/uploads/d8dd6e1d0dc45a788e869cdcdc99e178/Screen_Shot_2015-07-02_at_16.32.39.png)

![Screen_Shot_2015-07-02_at_16.33.18](https://gitlab.com/gitlab-org/gitlab-ce/uploads/7dbfe8b0ae229c32a08d6c7442976d83/Screen_Shot_2015-07-02_at_16.33.18.png)


See merge request !927

Without this link, the SAML docs are hard to discover.

Improve BitBucket integration documentation [ci skip]

This MR will improve the BitBucket integration documentation by

- mentioning the need to reconfigure GitLab after changing the configuration file, if you're using the omnibus package.
- adding a section about configuring a Host entry for bitbucket.org.
- switching additional steps 1 and 2, because connecting to BitBucket will only work after you added the SSH key and the Host entry.
- using sudo for the ssh connection in step 2, so that it is more obvious, that the git account should be used to establish the connection.

See merge request !670