Enhance Makefile

[candidtim]

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 This is necessary because it turns out that usb-serial-for-android jars are no longer updated/distributed for plain download, and project's README actually recommends to "link" its sources upon build directly - what this update effectively does.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.