Fixing module detection and autodiscovery of modules in maven project

Fixes for #25 "Cannot specify module name when creating a maven based GWT project" 
* fix: getModules() won't return a non-existing module
* [fix] NPEs in NewServiceWizard, when there are no modules available
* Support autodiscovery of modules in maven-projects == #27 (this fix is also included)

Please review.