Starting to flesh out the command line interface for the jpm executable

This commit includes some code to rnu the `jpm` CLI in the same Ruby process,
which will allow us to mock things out properly instead of creating a fake
JENKINS_HOME on the file system, yay
Implement all of `jpm list` with a simple text only output format

This brings over some of the initial plugin listing code from the
puppet-jenkins module
Add the MIT license for the gem