Fix issues with bp_core_time_since().

Previously, when the $newer_date parameter isn't passed, we generated a
UNIX timestamp using strtotime() and bp_core_current_time().  strtotime()
uses the default timezone, which can cause timezone discrepencies if the
server time is not UTC.

This commit removes the strtotime() usage and refactors bp_core_current_time()
to accept current_time()'s 'type' parameter.  This allows us to correctly
grab the UTC timestamp when setting bp_core_current_time()'s second
argument to 'timestamp'.

Commit also adds some unit tests.

Fixes #4310.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8676 cdf35c40-ae34-48e0-9cc9-0c9da1808c22