Create active directory account

This is the complete feature branch for creating a user account in active directory. This is not for merging, but for discussion.

This is the first and base feature, on top of which other features such as "password reset" and "staged account creation" will be built.

To set up the branch, just edit "active_directory.yml:development or :production" and specify the host, username and password for active directory.

This branch uses "net-ldap" and "ladle" gems. Ladle is a lightweight embedded instance of active directory used for testing with rspec. In this feature, I only used "ladle" to return a live connection to active directory for testing purposes.
 
Apart from adding the active_direction creation files, related gems and a related schema entry, there should be no other changes in the code.