READY: Use bundler gem conventions

Conversion to bundler gem conventions. Basically, 3 changes:

1. Gemfile points to ploy.gemspec -- which now includes both runtime and development dependencies.
2. Adds ploy/version.rb file, which is the "Ruby" way of versioning.
3. Adds a Rakefile for easy gem management. `rake install` will build and install your local changes to system, which is great for testing. `rake release` will push a tag up to GitHub and release a new version of ploy on rubygems.org.