Allow plugins to bail out of saving a follow relationship into the DB.

Use the 'bp_follow_before_save' hook and redeclare the 'leader_id' property
to zero if you want to bail.
Add ability to sort following and followers query by DB column.

This commit:
- Introduces new protected static method - get_orderby_sql().
- Adds a third parameter to get_following() and get_followers() methods.

See #53.
Add new dropdown filters - 'Newest Follows' and 'Oldest Follows' on member follow pages.

This commit:
- defaults the sort order on member follow pages to newest follows instead
  of last active status
- also adds dropdown filters - 'Newest Follows' and 'Oldest Follows' on
  member follow pages.

See #53.
Merge member scope filter functions.

Previously, there were two functions dedicated to filtering the member loop.
One for user pages and the other for the member directory.  This commit
merges these functions into one.

See #53.