Add xhprof_frame_begin & xhprof_frame_end

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