GroupMember cmdlets and tests. Refactoring

* All GroupMember related cmdlets are now implemented, including tests.
* The User cmdlets now support the parameter alias "Name" for "UserName"
to allow better pipelining (e.g. with results from Get-ODSGroupMember)
* Group related tests share functionality for adding and auto-removing
groups on cleanup.
* The binary is now added as module, not as PSSnapIn. This is easier and
no admijn rights are required in order to run the tests
* Updated README to be more useful