LDAP sign-in

This resolves #347 by adding LDAP user authentication to Barkeep. It's documented, with examples, in `doc/ldap.markdown`.

Here's the provider selection page. If only one provider of any type is specified, Barkeep skips this page and redirects immediately there, as before. Note that the names are specified arbitrarily in the configuration; I could've used "barkeep.com" instead of "Active Directory".
![barkeep_provider_selection](https://f.cloud.github.com/assets/153469/62475/28cfcb84-5d1c-11e2-972a-6e30bf14416a.png)

And here's the actual LDAP sign-in page. I'm not great at making things pretty, but it's functional.
![barkeep_ldap_signin](https://f.cloud.github.com/assets/153469/62479/850f01d0-5d1c-11e2-904a-9314c8d02cbd.png)

I'd like to test it with [Ladle](https://github.com/NUBIC/ladle), but there seems to be a version conflict between Fezzik's version of Open4 (>= 1.3.0) and Ladle's version (~> 1.0.0). I don't know Ruby best practice in these situations, so I'll leave it alone for now.

That said, I've tested it manually against OpenLDAP and Active Directory. However, the more exotic (read: anything other than `:method => :simple`) are untested. I'm just too distracted to set up anything more complicated right now.

Best,
Mike