make remove/delete consistent between container/image

PR for #89

To delete a container the method is `Docker::Container#delete`. To delete an image it's `Docker::Image#remove`. The docker API HTTP method for both operations is "DELETE", and the documentation describes both as a remove operation. While I don't think it matters much what the method name is, I do think it should be consistent between both containers and images.

This changes it so that `#remove` is the primary method, with `#delete` as an alias.