Introduces meta_query paramater for bp_has_activities() stack

This will allow plugin and theme authors to filter the contents of the activity
stream based on information stored in the activitymeta table, using the
powerful meta_query syntax familiar from the 'meta_query' WP_Query param.

Also adds unit test for 'meta_query' in BP_Activity_Activity::get(), and
integration test for bp_has_activities(). These tests are primarily designed
to ensure that our param gets passed properly down the stack; we rely on WP
to get the meta_query logic right.

See #3521

Props ericlewis

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6948 cdf35c40-ae34-48e0-9cc9-0c9da1808c22