Support LDAP for authentication

[raucao]

Over at https://kosmos.org we are using LDAP for user accounts. We would like to eventually deploy Armadietto for our users, and I think it would also be useful for anyone else using LDAP already, if they could configure Armadietto to add remoteStorage support to their existing accounts.

(In a perfect world, this would even be done by email providers, so that your email address is the same as your RS user address. We also plan to add an optional email service to Kosmos accounts eventually.)