Add Push::setCollapseKey() and ::setPriority()

[Philipp91]

Fixes #57 and #48.

Note that this PR ist rebased on top of #59, so need to merge that one first.

[AndyGaskell]

Hi @rolinger Did you have a look at this PR? There is quite a lot going on, but I think it looks ok.

[rolinger]

I have not. Haven't had the time for quite some while now. Feverishly getting new version of my app completed to meet some client needs coming up this month.

[rolinger]

If your good with it go ahead and merge.