Added improvements to Augeas module/state

Pull request for #7610

This improves the Augeas module and state in the following ways:

* Deprecated setvalue state in favor of change state
* Implemented lenses support
* Added more documentation

It also gives a significant performance increase, as explained in [this comment](https://github.com/saltstack/salt/issues/7610#issuecomment-42751674).

This also solves #7175, but only if a context is given (see the provided documentation in this PR).

I tried to provide a decent amount of documentation, but I don't know how to actually view it in the browser. Is there a script to compile the pages or something?