DRAFT: Implement an LDAP Target

[jonathanmaw]

This branch is unfinished due to lack of time.

Broadly, it tries to implement a way to take a ModelDifference and write all the necessary changes to that server.
It is configurable to skip certain operations (e.g. if adding new users is more complicated, and requires a separate target)
it still requires:

• Figure out decent default behaviour for adding a user (possibly with some kind of configurable template).
• Implement updating user's membership of groups (this looks a bit tricky, comments are included in this pull request for what I've figured out about how to do it)

[jonathanmaw]

Checking in on this pull request, an LDAP target is currently low priority, so will be picked up at a later date.