Using gradle to build COOJA

Why using Gradle ?
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- Can download dependencies dynamically (so we can delete lib that are bundled with contiki)
- Can perform check style (Android style guide)
- Can perform static analysis (FindBugs plugin)
- Cross platform
- Modern IDE (Eclipse, Idea IntelliJ,...) understands it
- Don't use XML as a build definition
- Gradle is used to build up Android application (http://www.gradle.org)

Let's use both ant and build.gradle. They can live together without any problems :D But gradle got much more features and I think in the long term should be preferred.
OK, I will change that.