Add "new_groupblog_post" activity type to "Posts" dropdown filter option.

When the "Posts" option is selected in the activity dropdown filter, it
only filters activity items by blog posts and not groupblog posts.

This commit allows both types of blog posts to be filtered in the activity
loop when the "Posts" activity filter is selected.
Only add 'new_groupblog_post' activity type if it doesn't exist.

Sanity check.
Return activity filter as querystring instead of array.

Plugin devs might expect the AJAX activity arguments as a querystring
instead of an array.
Update phpDoc for bp_groupblog_override_new_blog_post_activity_filter().
Merge pull request #33 from r-a-y/posts-activity-filter

Add "new_groupblog_post" activity type to "Posts" dropdown filter option