-> Always use javaCStringToUtf8 to convert byte array to String

->fix the NULL and 0 pointer comparisons mixes

- add an internal specific JNI function (javaCStringToUtf8()) to perform the UTF-8 conversion
- the SDK is configured to enable/disable the use of javaCStringToUtf8()

- return code is a basic error code (0 OK, -1  KO)
Remove TODO comments

- New class CommonSerializeUtils: helper class dedicated to serialization, to contain common code to all the OlmXXX classes that require to be serialized
- make OlmXXX classes extending CommonSerializeUtils

- OlmException class extends now from IOException
- update corresponding serializing unit tests
 - update