Start a .gitignore file

Ignoring:

 - .ruby-version  Used by rbenv
 - .bundle/       bundler state directory
 - pkg/           Where the PMT builds modules
 - vendor/        Common path for bundler gems
Add Gemfile for development environment

Brings down everything required and pins RSpec and Mocha versions such that
they follow what is used by Puppet 3.x.

Also add support for a `Gemfile.local` which can be used to customize the dev
environment. This file is tracked by `.gitignore`.
Add unit tests for r10k::config

WIP