first draft implementation of gradle build system

Hello,

I've implemented a first draft implementation of gradle for the AxonFramework.
just checkout gradle branch and run ./gradlew build (*nix) or gradlew build (windows)

Some remarks:
* some unittests failed due to timeout restrictions on the unit tests (probably need to invest in new hardware :-( )
* some unittests failed due to missing classses in the persistence xml (don't know what the reason is)

Current limitations:
* documentation module is not yet implemented
* there's only one build.gradle file. maybe it should be considered to split them for each subproject
* releasing is not yet implemented (although this can also easly be done in gradle)

I want to further improve the gradle build. But first want to check your opinion/remarks on this topic.

With kind regards,
Frederic