Figured since a JS Timestamp class was being implemented
it might be appropriate to create a complementary PHP
Timestamp class. Much of code is taken from wfTimestamp().
Note: Another change (I53dcf547) is attempting a related
issue. Not sure how much of scope overlap there is.
Change-Id: I68eb9f27eebe80df757187f634392e1bcba5551f
Signed-off-by: Tyler Romeo <tylerromeo@gmail.com>