Added support for disabling standard login

Only enabled authorization methods are displayed on the sign in page. If no authentication methods are enabled, an appropriate message is shown.

This pull request is in reference to a feature request that was for only allowing login via LDAP. The feature request can be found here: http://feedback.gitlab.com/forums/176466-general/suggestions/3788023-only-allow-login-via-ldap.

The original issue that brought this up is issue #605. It has since been closed due to being moved to a feature request.

Screenshots of each possible scenario are attached.
![ldap_and_standard](https://f.cloud.github.com/assets/2180127/1442797/6830bf9e-41c9-11e3-8165-e78ff54fe9e7.png)
![ldap_omniauth](https://f.cloud.github.com/assets/2180127/1442798/683cac8c-41c9-11e3-98c8-a8945bc93009.png)
![ldap_only](https://f.cloud.github.com/assets/2180127/1442799/683db758-41c9-11e3-8adc-2a04867756c5.png)
![ldap_standard_omniauth](https://f.cloud.github.com/assets/2180127/1442800/683e25d0-41c9-11e3-8eb8-b6b38e126fd4.png)
![no_auth_methods](https://f.cloud.github.com/assets/2180127/1442801/684022b8-41c9-11e3-8966-8cd112aa03ee.png)
![omniauth_only](https://f.cloud.github.com/assets/2180127/1442802/68423940-41c9-11e3-9a25-8502c88a2257.png)
![standard_omniauth](https://f.cloud.github.com/assets/2180127/1442803/6846a674-41c9-11e3-8d25-68a6a460d0b2.png)
![standard_only](https://f.cloud.github.com/assets/2180127/1442804/6846ea62-41c9-11e3-8158-940076b52870.png)