31028 java desktop build

This PR splits the project into 2 jars, one core and one Android-specific.

The build is achieved with 2 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.
Thanks for spotting that, OK it looks like we'll have a 3rd jar just for j2se stuff, this should be quite easy.