Add ability to run statprof as a module

This supports the following usage pattern, similar to other profiling tools, or pdb:

    $ python -m statprof myscript.py
    <output of myscript here>
     time    seconds   seconds  name
     93.67      0.20      0.20  myscript.py:8:main
      2.53      0.01      0.01  myscript.py:7:main
      1.90      0.00      0.00  myscript.py:9:main
      1.27      0.00      0.00  myscript.py:6:main
      0.63      0.00      0.00  myscript.py:10:main
      0.00      0.21      0.00  runpy.py:162:_run_module_as_main
      0.00      0.21      0.00  myscript.py:18:<module>
      0.00      0.21      0.00  statprof.py:471:<module>
      0.00      0.21      0.00  runpy.py:72:_run_code
      0.00      0.21      0.00  statprof.py:468:main
    ---
    Sample count: 158
    Total time: 0.210000 seconds