FCM support (refactored)

[TysonAndre]

Based on PR #139

This uses anonymous field (in a manner similar to subclassing, but very different) to factor out code common to GCM and FCM http://dennissuratna.com/using-anonymous-field-to-extend-in-go/

• had duplicated code, which would double the work on future changes to GCM/FCM modules and possibly lead to inconsistencies/bugs.
• I recently set up android studio with the FCM mode from https://firebase.google.com/docs/cloud-messaging/android/first-message , and I'm working through the tutorial. This will be tested with android devices, GCM on iOS/other may or may not need more work Still need to manually call /addpsp, /push, and /subscribe on a local instance of uniqush to test this manually
• Projects used to test this: https://firebase.google.com/docs/cloud-messaging/android/first-message https://github.com/firebase/quickstart-android/tree/master/messaging (Getting the token from logcat output)
• The cloud_messaging base class will probably be refactored in future changes, but tests shouldn't need changing

[TysonAndre]

This works in the emulator, and the way to set up uniqush for FCM is pretty much identical to GCM.

To set up FCM, Log in to https://console.firebase.google.com

1. Click on settings (The gear icon)
2. Choose the cloud messaging tab
3. The projectid (called sender id in FCM) should be visible in the project credentials section. So should the apikey (Called "Server key" in FCM). Google recommends the longer "Server Key" over the legacy server key.

# Add the PSP, can call the service anything, e.g. myproject.fcm,
# or group it under another service
curl http://127.0.0.1:9898/addpsp -d service=myproject.fcm -d pushservicetype=fcm -d projectid=<project id>

# Add a test  subscriber programatically by calling the rest API
# E.g. https://github.com/firebase/quickstart-android/tree/master/messaging
# is a test project using FCM which gives you the id in logcat output
curl http://127.0.0.1:9898/subscribe -d service=myproject.fcm -d subscriber=uniqush.client1234 -d pushservicetype=fcm -d regid=blahblah:abcdefgabba_restofregid

# The below command sends a push over FCM
# (See the documentation for GCM, the params are almost the same.
#  The documentation can be found at http://uniqush.org/documentation/usage.html)
# The query param uniqush.payload.fcm can also be used instead to provide raw
# JSON payloads for the FCM module to send.
curl http://127.0.0.1:9898/push -d service=myproject.fcm -d subscriber=uniqush.client1234 -d msg=hello