Implement Channel API

[gabriel-laet]

Implement dl-api's Channel feature:
https://github.com/doubleleft/dl-api-android/blob/master/lib/dlapi/src/main/java/com/doubleleft/api/Channel.java

[endel]

Maybe use this dependency: https://github.com/aslakhellesoy/eventsource-java