Add functions to make the difference between the native and the java code version.

Factor out the version management in the makefiles.

- Modify constructors for inbound and outbound group sessions
- Add new Ecxception class

 - new file olm_utility.cpp to have a stand alone function to initialize/alloc a random buffer
 - new class OlmMessage
 - complete OlmSession API with encryptMessage()
 - comments review
 - OlmAccount unit tests are green
 - new gradle to compile the shared lib according to debug mode

- Add Android Studio project