procfile: Improve testing of Procfile class

And move Procfile and parse_procfile into honcho.environ.
command: Use argparse's subparser dest attribute

This uses the 'dest' attribute of ArgumentParser#add_subparsers to store
the desired command, thus avoiding nasty traceback spewage on Python 3
due to this change:

    http://bugs.python.org/issue9253#msg186387
test: don't monkeypatch Process if not checking coverage
command: Move process creation logic under test

The logic associated with how many of which process to create, how to
increment ports, and so on, was largely untested.

This commit moves the bulk of that logic into the
`honcho.environ.expand_processes` function, which has a full suite of
unit tests.