Adds support for jar files (in order to create webjars)

See http://webjars.org. Create a webjar like this:

    var artifactOptions = {
        groupId: 'com.example',
        artifactId: 'example-artifact',
        version: '1.0-SNAPSHOT'
    };

    maven: {
            options: artifactOptions,
            install: {
                options: {
                    goal: 'install',
                    packaging: 'zip',
                    type: 'jar',
                    injectDestFolder: false,
                    url: 'file://a-repo'
                },
                expand: true,
                cwd: 'target/',
                src: ['**/*.*'],
                //dest: '/META-INF/resources/webjars/' + artifactOptions.artifactId + '/' + artifactOptions.version
            }
        }

Also a small refactoring to avoid duplicate code. Does not add unit tests for the jar part (that would require some work restructuring the unit test approach)