Removed the `get`, `post`, `put`, and `delete` methods from
Docker::Model::ClassMethods.

I think these made the class definitions less clear -- now everything must
use `request`.
Simplified `Docker::Image.create_tar`.

Since there was no way that I could find to simply pack a String into a
tar using `Archive::Tar::Minitar`, I used the tar writer provided by
rubygems instead.
Version bump 1.1.1
Merge pull request #14 from swipely/clearer-class-definitions

Clearer class definitions