Replace `BP_PLUGIN_DIR` and `BP_PLUGIN_URL` constant usages with `plugin_dir` and `plugin_url` BuddyPress variables, respectively. Also replaces `$bp` global touches with `buddypress()` calls within the above scopes.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7756 cdf35c40-ae34-48e0-9cc9-0c9da1808c22