jpi: new plugin tools command line

[Charles Lowell]

Quick update: As part of the effort to replace the rake tasks with a command line tool, I've add the `jpi` executeable to the jenkins-plugin gem, and ported the :server Rake task over to the CLI.

This means that if you grab jenkins-plugin >= 0.1.7, then you can start the server in the ruby plugin via

jpi server (or just `jpi s` for short ala rails)

There are stubs for the other commands  (new, generate, build, etc...) which should provide a way forward even if they don't work yet.

README updated @ https://github.com/cowboyd/jenkins-plugin

cheers,

Charles