Modify BP_FOLLOW_URL to play nice with symlinked environments.

plugin_dir_url() doesn't work properly when the BP Follow directory is
symlinked.

This commit switches the BP_FOLLOW_URL constant to use plugins_url() with
basename().  I'm not sure why WP doesn't use PHP's native basename()
function; it might have something to do with IIS installs.  Something to
keep an eye on.
Introduce new function - bp_follow_is_doing_ajax().

Since we're still supporting BP 1.5, we can't use the DOING_AJAX constant
to determine if AJAX is running.  To workaround this, we check the server's
HTTP_X_REQUESTED_WITH variable.

Replace older inline references with this new function.
Reduce queries when BP_Follow_Component is loaded in the admin area.

- Don't load logged-in user's follow counts if we're in the admin area.
  This is unnecessary since this data isn't being used anywhere.

- Don't load the Buddybar code in the admin area when the WP Toolbar is in
  use.  If the Buddybar isn't being used, we don't need to load it up in
  admin area.

Note: We check if we're in the admin area by seeing if WP_NETWORK_ADMIN is
defined.  This is done because is_admin() sees admin-ajax.php as an admin
page.  This doesn't work for our purposes.
Introduce updater mechanism.

- Adds a 'revision_date' property to the BP_Follow_Component class
- Includes new class - BP_Follow_Updater - which handles installing /
  upgrading the DB table and saving the revision date to WP options
- Updates unit test installer to use the new updater method
Remove older activation hooks.

Commit c1b01e9 handles installing the DB table using a different approach.
Therefore, this commit removes the older register_activation_hook() method.
Remove unneccessary return call.
Braces and spaces!
Fix trailing slash in BP_FOLLOW_URL define.

See commit 10d06c6.
Fix incremental count when unfollowing user on members directory.

Commit 8a414c1 broke the incremental count on the members directory. This
commit fixes this.

See #14.
Add versioning to javascript to bust cache.
Add support for the new "follow_type" DB column.

An updater was added in commit c1b01e9 to upgrade the BP Follow DB table to
add a "follow_type" DB column.

This commit fills in all the places where the "follow_type" can be used.
First pass at following sites.

This feature is only supported in BP 2.0+ and when WP is in multisite mode.
BP must also be network-activated.

Specifically, this commit:
- Adds a "Sites I Follow" tab to the activity directory
- Adds a "Sites I Follow" subnav item to a user's "Sites" page
- Adds a follow button in the blog loop to follow / unfollow a site
- Adds a follow button in the footer of each site to follow / unfollow a
  site.  This button currently mimics Tumblr's interface.

Todo: Use javascript to follow / unfollow a site.  UI feedback for the
footer button.

See #31.