31028 java desktop build

This PR splits the project into 3 jars, one core, one Android-specific, one Java SE specific

The build is achieved with 3 gradle sub-projects, which might make importing into Android Studio a bit more tricky.

The android project currently only contains com.cloudant.sync.sqlite.android.AndroidSQLite and com.cloudant.sync.sqlite.android.AndroidSQLiteCursor. Also introduced is a bit of dynamic class-loading (Class.forName), for loading the right SQLDatabase implementation and dynamically fetching Android run-time info without making a hard dependency on these classes.

Also updated are the README and sample project, which has been successfully tested on the emulator.