Enable profiling via xhprof

Enable profiling via xhprof

Add a helper class to assist in collecting profiling information using
XHProf <https://github.com/phacility/xhprof> and a Profiler
implementation to hook it into the existing MediaWiki profiling system.

The Profiler includes support for generating tabular reports similar to
the traditional output of ProfilerSimpleText and ProfilerSimpleTrace or
sending data to a udpprofile daemon as supported by ProfilerSimpleUDP.
It also produces data compatible with the debugging toolbar.

Bug: T759
Change-Id: I16a75cb7636cb5dcef3830d738b2dcd2047d0aaa