Follow Members: Do not query for button if on own profile.

Gets rid of a query when a user is on their own profile page.
Follow Blogs: Fix loop filtering depending on the user ID.

Props @christianwach.  See #31.
Add actions so plugins can determine when BP Follow is loaded.

Hat-tip @christianwach.
Follow Blogs: Remove set_activity_score_on_activity_directory() method

This class method interfered with the 'bp_activity_screen_index' hook,
since this hook was used both as an action and a filter.  This is a bug
in WP core as it cannot distinguish between the two contexts when the hook
is named the same.

This functionality isn't being used anymore anyway in follow-blogs, so
we're removing it here.

Hat-tip @christianwach.

Fixes #47.