Introduce new LDAP share with subgroups possibility.

In a common scenario i want to have some users (top level users like
employees) who can share with users some levels beyond them.

F.e.

We have Employees, Teachers and Students.
The Employees want to be able to share with Teachers and Students but
the Teachers and Student should not be able to share with the employess.

The possibilities wich are given now (a setting to share with everyone
and a setting to only share with members of the same group) are not what
we needed.

We needed a Possibility to create a LDAP (MSAD) Structure with groups,
wich contain other groups as members. So we can create a hierarchical
group structure.

OwnCloud wasn't ready yet to handle such a structure. So we implemented
it.
We would appreciat, if you could review it, and if you want it, maybe you can
implement it in the main branch.

If something isn't like you guys want it, please let me know.
I would love to make all changes like you wanna have it.