Add support for server-side ordering

[deveiant]

Ruby-LDAP has sortattr and sortfunc arguments for LDAP::Conn#search* methods, but they sort results after they've returned to the client, not on the server. This means that using a limit is only useful if you want objects in whatever order the server returns them to you, which is almost never the way a human expects them to be sorted.

There is, however, a  Server Side Sorting control which can be used to sort results before they're returned.

We'll likely implement this in terms of the controls mechanism described by #2.

[deveiant]

Committed in r244.