Sconscript: refactoring

Rename variables, to distinguish the lib's name from the name of the
lib's variable.
Sconscript: versioning java and python libs
Sconscript: get version from git

* If GitPython module is available on the python environment, get
version of AvTranscoder from the last tag on github repository.
* Else AvTranscoder's version is 0.0.1.
Sconscript: manage java binding if first build

* We can't generate JAR file while first build, because java classes are
not already generated. Need to relaunch the build to generate it.
* Print a warning message for java binding if first build.
* Generate JAR file when it's possible.
Sconscript: add version to the JAR name
Merge pull request #59 from cchampet/build_versioning

Build: versioning java and python libs
Get version from git