Separate Java SE and Android build artifacts.

This allows developers to use our libraries on Java SE or Android platforms without bringing in
unnecessary extra dependencies.

There are now three sub-projects:

* sync-core
* sync-android
* sync-javase

This results in the following jars being published to the maven repo:

* cloudant-sync-datastore-core
* cloudant-sync-datastore-android
* cloudant-sync-datastore-javase