Make mpet a package

[Raymond Smith]

Organize so that mpet/ is a directory that is in the path while in the working directory.

• Provide a script (runmpet) that either be in system path (when installed, e.g. with pip) or in working directory that parses command line arguments and passes them to mpet/mpet.main.

• In readme tell user to copy default configs and runmpet to their working directory.

[Raymond Smith]

Addressed in pull request #48.