Improved Logging

I really didn't like the way we'd implemented logging so I did some research and re-wrote it.

- Using [Yell][1] - This gives us an improved base logger with the capability to log in custom formats more easily (perhaps JSON for shipping?) via adapters. It's also a very lightweight dependency (no additional deps)
- Created Shrinkwrap::Logging module to be included in classes that need logging.

```ruby
module Shrinkwrap
  class Stuff
  include Shrinkwrap::Logging
    def initialize
      log.debug "Shrinkwrap::Stuff is being initialized"
    end
  end
end
```

feels like a much more comfortable interface than having to prepend Shrinkwrap.log to every logging call.

[1]: https://github.com/rudionrails/yell