Theme compatibility

Hi foxly and gang,

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](https://github.com/BP-Media/bp-album/blob/master/includes/templates/album/pictures.php)
* [/members/album/single.php](https://github.com/BP-Media/bp-album/blob/master/includes/templates/album/single.php)

Since BP 1.7 opens up BP to all themes, this doesn't work so well.

Try:

* Installing BP 1.7
* Activate BP Album
* Switch the theme to Twenty Twelve (or anything other than bp-default)
* And navigate to a user's `/members/USERNAME/album` to see what I mean.

What we do in this commit is:

1. Add BP 1.7 theme backpat functions so we can use them in BP Album
2. Allow themes that are already using these older templates to continue working as-is
3. Change BP Album to use /members/single/plugins.php as the template so we can inject templates into BP without changing the surrounding markup
4. Re-route older template locations to use our new templates to avoid conflicts.

   New templates reside in:
   `/buddypress/members/single/album`

Everything should be backwards-compatible down to BP 1.5.

Read the commit messages and inline doc for more details.

Once you've tested out the pull request, let me know if you guys have any Qs.