Added a Gradle script and removed Ant script

I converted the project to use Gradle, which is a lot better than Ant/Maven. You can install Gradle from http://gradle.org. Just run the gradle command inside of the MapDB folder to build a jar, and afterwards, you can get the JAR from build/libs. To generate a javadoc you can run: gradle javadoc. Afterwards, build/docs/javadoc will contain your javadocs.

Hopefully this will bump up productivity for you, and help you at the same time.