Support for running Applications under cProfile.

Simply pass `profile=True` to `Application.run` or `quickstart`. The
results will be displayed when the program exits.
Sort profile stats by tottime.
Allow profile stats to be written to a file.

If you provide a file path in the `DIESEL_PSTATS` environment variable
the profile stats will be written to that file for later analysys
instead of printed to stdout.
Toggle profiling with DIESEL_PROFILE environ var.

No need to make an API change, and this makes controlling both the
toggling of profiling and the selection of a stats path consistent (use
the environment for both).
Merge pull request #52 from dowski/app-profiling

Support for running Applications under cProfile.