Add a create method for instantiating a generator in unit test

In a unit test, you sometimes need to test some methods of a generator without actually running it.
The new method `helpers.create`, will do exactly this, and offers the same api than the existing `helpers.run` method.

One possible scenario is that you want to unit test a custom base generator.
With this PR you can do:
```javascript
it('add() is working', function(done) {
        var runGen = helpers.create(path.join(__dirname, '../custom-generator'))
            .inDir(path.join(os.tmpdir(), './temp-test'));
        this.runGen.on('ready', function() {
            var retval = runGen.generator.add(12, 13);
            assert(retval === 25);
            done()
        }.bind(this));
    });
```