added notification ability fixes #9

[AdriVanHoudt]

fixes #9 according to https://developers.google.com/cloud-messaging/concept-options#notifications_and_data_messages

I added the option at the end to make it non breaking (I hope, never done Go before ^^)
Let me know if this needs any changing otherwise would it be possible to merge since we need it asap.

[AdriVanHoudt]

Hate to bump this but any eta on this or comments?

cc @alexjlockwood

[0x7061]

@AdriVanHoudt Can you add content_available and priority? See #12 - I think your PR is already more advanced.

[AdriVanHoudt]

I can look into it, content_available is ios specific right?

[0x7061]

Yes! It's needed to wake up an iOS app if a notification arrives. Thank you very much!

[AdriVanHoudt]

I can only do this by end of the week btw, just a heads up ;)

[0x7061]

That's absolutely fine :)

[pauldub]

Hello, it would be nice to see this being merged. It is simple enough that it should not be an issue @alexjlockwood would your like a PR with Notification being pointer or anything else? I think it would provide a better API that way.

[AdriVanHoudt]

@pauldub wow completely lost track of this, will add this back on my todo list!