First pass at bp-notifications component. Includes:

* Backwards compatibility for old core functions.
* Screens, functions, classes, and actions for Notifications.
* Improved class methods for getting, updating, and deleting notifications.
* Template parts in bp-legacy for theme compatibility.
* A few basic unit tests.

@todo's:

* Improve template output with dedicated functions, markup, classes, et all.
* More unit tests.
* Pagination links.
* Auto-activate on update, so existing installations do not lose previously core functionality.

See #5148. Props johnjamesjacoby, boonebgorges, r-a-y.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7521 cdf35c40-ae34-48e0-9cc9-0c9da1808c22