Add xhprof_frame_begin & xhprof_frame_end

[bd808]

Add functions for sub-function profiling.

• xhprof_frame_begin : Starts an artificial frame. Together with xhprof_frame_end, this times one block of code execution as if it were a function call, allowing people to define arbitrary function boundaries.
• xhprof_frame_end : Ends an artificial frame that xhprof_frame_begin started. One has to make sure there are no exceptions in between these two calls, as otherwise, it may report incorrect timings. Also, xhprof_frame_begin and xhprof_frame_end have to be paired up really well, so not to interfere with regular function's profiling, unless that's the intention.
• XhprofFrame : Wrapper object that calls xhprof_frame_begin in its constructor and xhprof_frame_end in its destructor.

This functionality was back ported from HHVM's internal XHProf implementation. There has been one small change from the HHVM implementation with the addition of the optional $wrapped boolean parameter to xhprof_frame_begin which is used to fix a bug in the upstream XhprofFrame implementation which mis-attibutes function calls in a frame introduced via XhprofFrame to XhprofFrame::__construct rather than the name provided for the frame.

Fixes #51

[bd808]

Ping

[bd808]

:birthday: I meant to ping on 2015-11-28, but better late than never as the Hallmark® cards tell me.