Added `generate` task to Gradle build script.

I've added `project.group` as it probably should be defined in anyway.
`mainClass` replaced with `mainClassName` to mimic the Application plugin property.

I've set default log level for `generate` task to `INFO`. This is different from `grainw` behavior, but running Gradle manually allows to easily change the verbosity by supplying -q, -i and -d command-line arguments.