Implement `honcho run PROCESS`

This makes `honcho run` primarily about running a Procfile-specified process in the foreground (the first proposal at https://github.com/nickstenning/honcho/issues/56#issuecomment-34546939). Previously it was about running an arbitrary shell command, which still works, but now as the fall-through case if a single arg to `honcho run` doesn't match a Procfile process.

The reason this change is desirable is to be able to use debugging tools such as pdb along with Procfile process aliases (#56). Furthermore, this brings Honcho back into harmony with Foreman's behavior, but in a way that is documented more sensibly (Foreman presents the process case as the exception rather than the command case).

This change is backwards-compatible except for edge cases where a shell command name was used as a Procfile process name. So, for example, if `mv` was a process name, then calling `honcho run mv` would now invoke the Procfile process rather than the shell command. Since we also check for the number of arguments when trying to interpret as a process name, this edge case is especially edgy. I think this can safely be considered a backwards-compatible change for all practical purposes.

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P.S. For Gittip folks: I'm introducing this fork on Gittip in https://github.com/gittip/www.gittip.com/pull/2384. If/when you change it here's how to revendor it:

 - Go into a local working copy of the fork.
 - `echo __version__ = "'$(git rev-parse --short=8 HEAD)'" > honcho/__init__.py`
 - `python setup.py sdist`
 - `mv dist/honcho-*.tar.gz ../www.gittip.com/vendor/`

Then go back over to `www.gittip.com`, remove the old version and update requirements.txt.