Add Delete method to Pusher

[beorn7]

Fixes #611.

@nnoto what do you think?

This allows

push.New("http://localhost:9091", "some_job").Grouping("foo", "bar").Delete(),

[nnoto]

Looks good to me, much appreciated!

[beorn7]

OK, I'll merge. You can use it by building from head. I'll see what else is in the pipeline to decide when to cut a release.