Feature gradle

This PR adds support for gradle on a basic level.
To start the application with spring-boot you have to run `gradle bootRun`.

If you want to use the prod profile the command is `gradle bootRun -Pprofile=prod`

`gradle build` builds the Project and the resulting jar can be found in `build/libs/jhipster-0.1-SNAPSHOT.jar`.

The gradle plugins for grunt, gulp and nodejs are used to run the frontend build.