Fix BP_Group_Extension methods for strict standards
Readme
When deleting a groupblog post, delete corresponding activity

We need our own function to delete groupblog post activity, because
bp-groupblog modifies the default values of the activity type, item_id,
and component, making BP's native functions not work for this purpose.
Readme update
Bump version numbers to 1.8.5
Merge branch '1.8.x'