Using Proguard (or anything similar) to shrink compiler size

[mosser]

The compiler is around 10Mb in size, which is still OK but could be improved by shrinking it a little bit. So far the "jar-with-dependencies" plugin create a compiler JAR file that contains ALL the libraries linked, even if we're not using everything from them.

We might want to consider using Proguard (or anything similar) to provide static analysis of the code and remove unused classes from the final jar file.