Add ability to query activity items by date.

This commit:
- Introduces the `BP_Date_Query` class.  This class extends `WP_Date_Query`, which will enable us to easily add support to query various BuddyPress items by date.
- Adds a new `'date_query'` parameter to `BP_Activity_Activity::get()`.  This parameter utilizes the new `BP_Date_Query` class and is an example of how to use the class in BuddyPress.  (Will add support for the Groups component in a later commit.)
- Adds unit tests.

Due to the dependency on the `WP_Date_Query` class, this functionality is
only available in WordPress 3.7+.

Fixes #5803.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8787 cdf35c40-ae34-48e0-9cc9-0c9da1808c22