Letting send silent pushes

[nesimtunc]

I'm not able to test GCM but APNS. So, I would like to allow sending silent pushes for APNS. Because APNS (or iOS itself) sets default push sound if the sound property defined. Even you set an empty string or non-existing sound in the payload for the purpose of sending silent push.

[coveralls]

Coverage remained the same at 100.0% when pulling 2ac733e on nesimtunc:master into 5f21736 on appfeel:master.

[appfeel]

Have you tried to send the notification with a sound file that does not exist in the phone?

Something like:

sound: 'unexisting-everandever.unknown'

Is this breaking your app? I know it's not ideal solution but... your implementation requires to be ported to rest of platforms too before it can be merged: the main priority of this module is to provide an agnostic interface as much as possible. If there is an option that can be implemented in all platforms (which I suspect is the case of silent notifications), it should not be only implemented in ios, it should be implemented in all of them.

[appfeel]

And I'm sorry for telling you this now that the work is already done (I really appreciate contribs), but in fact it requires more work and maybe there is an easy workaround, this is the reason.

[nesimtunc]

Yes, it doesn't work that way as I mentioned. But, it's OK not merge into your branch. I already forked and started to use in my project. Thanks.

[nesimtunc]

Oh I just saw the other pull request which is about this also. Sorry for second merge requests. Thanks.