Support for LDAP authentication

This fork resolves issue #554 - https://github.com/NuGet/NuGetGallery/issues/554

For our internal enterprise NuGet Gallery, we required LDAP based authentication. This fork contains the bare minimum of that implementation and is configuration driven (if the web.config contains LDAP configuration, it is seamlessly used as the authentication mechanism, users are silently added to the NuGetGallery database on first login).

The implementation works out of the box but one feature yet to be implemented is the ability to quietly update the user password in the NuGet Gallery database whenever the LDAP user changes it.
UPDATE: User passwords are no longer stored.