Introduce support for a 'type' sorting parameter in BP_Group_Member_Query

The 'type' parameter supports any of the arguments ('newest', 'active', etc)
supported by the parent class BP_User_Query. In addition, it's possible to
sort by type 'last_joined' and 'first_joined'.

This changeset also introduces the 'type' parameter through the
bp_group_has_members() function stack.

See #921

Props imath, boonebgorges

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7948 cdf35c40-ae34-48e0-9cc9-0c9da1808c22