Add custom data fields for Android push

[renefloor]

As discussed in #1
It would be good when the possibility is introduced to add other fields than only message or title. In the Android push platform you can use any field you want.

[nickygerritsen]

Apple can also have custom fields.

Probably add a general setField($field, $value) function to the interface and implementations?

[renefloor]

Didn't know the implementation of Apple, than that makes sense yes.

[johnroyer]

In this document, custom field can be set in array data (the same as title and content). I think it is great to add a method setField(), or maybe setFields(['fieldName' => 'key', ...]).

APNS notification payload structure in HTTP2 version seems different from GCM. I can not find the old version APNS API document which using socket to send payload. Is this library plan to suppport old version of APNS API ?

[jszanto]

This has been resolved in 1.0.4 I believe. There are currently no plans to support older versions of the APNS API.