Add m2e configuration to run scijava-common's annotation processor automatically

We should use the technique [illustrated on the Eclipse wiki](http://wiki.eclipse.org/M2E_plugin_execution_not_covered#execute_plugin_goal) to execute the `EclipseHelper` class as soon as scijava-common is a dependency of the current project. That will make it painless to import SciJava projects into Eclipse and develop them, without having to jump through hoops.