Enhance Makefile

This pull request brings few changes to the Makefile, allowing users to rebuild the library from scratch after the clone:

1. `usb-serial-for-android` submodule is now initialized automatically (previously had to be initialized manually with `git submodule init && git submodule update`, before launching `make init`).

2. Compiles all sources altogether - `ArduinoFirmata-Android` and `usb-serial-for-android`, which ensures that compiler will find necessary depdendencies, and that `ArduinoFirmata-Android` is compiled against same `usb-serial-for-android` version as the one which will be then assembled (see below).

3. Creates both jars: for `ArduinoFirmata-Android` itself, and for `usb-serial-for-android`. This is necessary because it turns out that `usb-serial-for-android` jars are no longer updated/distributed for plain download, and project's README actually recommends to "link" its sources upon build directly - what this update effectively does.

P.S. I think though that a more proper way to manage the build process would be to implement a Gradle project, which will allow correct build on all platforms, and will allow managing `usb-serial-for-android` properly (as a dependency). This update however is a quick change to enable easier assembling of the libraries, while not changing the process conceptually.