Removed the call to waitpid in pfork

The call to waitpid, on OSX (tested on 10.7 and 10.9) block for _one_
    of the calls for about 2sec. This is usually on one of the first 3
fork
    IDs.

    As the previous reading from the pipe provides for the necessary
    synchronization the call to waitpid is redundant. Uncommenting for
now.
Added pfork_shm and tests for pfork/pfork_shm

pfork_shm provides a convenient way to use pfork while using one or
more shared memory variables for the return value(s)
Merge pull request #5 from rened/master

Speedup for pfork on OSX, adding pfork_shm for shared memory return values