Implement pause/unpause operations

Implementation is inspired by the existing stop/restart code, so four new methods exist:

> #pause! and #unpause! both perform the associated action and
> return the Container. #pause and #unpause do the same,
> but rescue from ServerErrors.

Increased API_VERSION to 1.12, since that is the earliest Doocker API version that these operations exist

This is a PR for #151