Convert project to use Gradle instead of Maven.

[gabizou]

All adapted features provided by the Maven configuration have been reproduced with Gradle. The new Gradle script also supports automatic artifact signing with GNU Privacy Guard keys. Included is the Gradle Wrapper that can be used by anyone to automatically build the project without having to install Gradle itself.

Advantages include:

• No more Maven! (At least for this project specifically)
• Easier to read build script than both Ant and Maven
• Universal support for cloning and building out of IDE
• Generated artifacts are still valid Maven artifacts
• Cleaner build process for CI systems.
• Automated License header application on demand (note the changelog)
• Easy to define new custom tasks in the Groovy language to perform other actions

I have tested this branch with the following as an example of the produced binaries:

http://ci.afterkraft.com/job/Open-MBEE-util/