General improvements

I have update my plugin on the gradle_build branch: [netbeans-gradle-project](https://github.com/kelemen/netbeans-gradle-project/tree/gradle_build). So, you might use this as a quick reference to some changes.

Most changes are related to specifing the content of the manifest but there are some others like up to date check changes and run/debug task (compatible with the NB plugin). Also, rather than expecting an "netBeansInstallDir" property, the run/debug tasks rely on "netBeansExecutable" to easily overcome different OS issues (also, there are multiple executables on Windows to choose from, so the user might decide which). Later in the NB plugin, I plan to add support for automatic configuration of "netBeansExecutable" if not specified.