Proguard : reduce the size of the binaries

DO NOT MERGE AS IS !
I'm creating this PR to [start the conversation](https://github.com/blog/1124-how-we-use-pull-requests-to-build-github)

I've been trying to integrate Proguard to reduce the size of the downloaded executable, to no success at the moment. I lack experience with Proguard to fix it. 

If anyone want to give a hand on this he is very welcome !