Add Firebase Messaging

[Freydoonk]

According to the changes of Google I change the PushSharp to work with FCM.
Firebase Cloud Messaging (FCM) is the new version of GCM. It inherits the reliable and scalable GCM infrastructure, plus new features!

[ForNeVeR]

Why did you reformat all the code? It sufficiently complicates the merge of your contribution.

[Freydoonk]

@ForNeVeR I do this to remove the extra using and convert space character to tab. It is the standard of code style that I used.

[ForNeVeR]

@Freydoonk just a suggestion: it's rarely acceptable to reformat the open source project code in scope of any PR without talking with maintainer first. It may be the code style you use, but it's definitely not the code style that's used in PushSharp codebase.

And that's not nice that you were converting the code style and updating some project settings in a single commit. Generally it's better to always completely split the formatting changes to another commit or even a separate PR. That will allow the maintainer to cherry-pick only the changes he want to include in the project.

[Freydoonk]

@ForNeVeR You are right, I consider your notice in future

[fproulx-pbox]

What's missing for this to be considered for merging to master ?
I would really like to use FireBase ASAP.

[h3smith]

No one is ever going to merge a request that changes every single file and every single line of virtually every single file.

The pull request has to have only the relevant changes, not formatting changes as stated above.

[ForNeVeR]

@fproulx-pbox actually you already can use FCM with PushSharp, see #752.

[knopa]

When be the nuget package?

[jilnesta]

When it available on the nuget package ? pls :((

[JotaBritoDev]

(https://developers.google.com/cloud-messaging/http-server-ref)

Could someone look ahead for this PR?