Introduce backpat functions for < BP 1.7.

This is so we can utilize BP's new template-loading approaches like the
template stack and use other functions like bp_get_template_part().
Reformat template loader.

Previously, BP Album would supply its own templates as a full template
(using get_header() / get_footer()) and not as a template part.

See:
/members/album/pictures.php
/members/album/single.php

Since BP 1.7 opens up BP to all themes, this doesn't work so well.

What we do in this commit is:

1. Allow themes that are already using these older templates to continue
   working as-is
2. Change BP Album to use /members/single/plugins.php as the template so we
   can inject templates into BP without changing the surrounding markup
3. Re-route older template locations to use our new templates to avoid
   conflicts.

   New templates reside in:
   /buddypress/members/single/album
Remove older templates bundled with BP Album.
Make sure bp_album_load_subtemplate() supports our newer templates.

Use bp_locate_template() to load templates instead of STYLESHEETPATH /
TEMPLATEPATH lookup.
Rename older template name to newer one.
Merge pull request #4 from r-a-y/theme-compat

Theme compatibility