Ruby Refactorings & Object Improvements

I know there are a fair amount of changes here, so I'll try and list them all off:

- Since I noticed Travis wasn't testing against 1.8 and since it has been sunsetted, I switched everything to use the newer 1.9 hash syntax
- All keys can now also be passed around as symbols & underscored strings, eg ``:status_code`` instead of ``'StatusCode'``. All JSON parsed objects also use symbolized-underscored keys
- All requests use the correct ``Docker::VERSION`` in their ``User-Agent`` header
- ``Docker::Image.build`` and ``Docker::Image.build_from_dir`` now take a block that will stream the build progress
- ``Docker::Image`` and ``Docker::Container`` objects now have attr_readers for all of the expected attributes after a search

This would required a major version bump (to 2.0.0) since there are some backwards-incompatible changes.