Added form factors generators

When we introduced the new Ginjector generator, it wasn't obvious as to how to do multiple device permutations based on gin configurations.
This is related to: https://github.com/ArcBees/GWTP/issues/158

I'm introducing a new gwt module that will let you bind a base module / per permutation.

Example:
```xml
    <!-- Form factor specific modules -->
    <set-configuration-property name="gin.ginjector.module.desktop"
                                value="com.gwtplatform.samples.mobile.client.gin.DesktopModule"/>
    <set-configuration-property name="gin.ginjector.module.mobile"
                                value="com.gwtplatform.samples.mobile.client.gin.MobileModule"/>
    <set-configuration-property name="gin.ginjector.module.tablet"
                                value="com.gwtplatform.samples.mobile.client.gin.TabletModule"/>

    <!-- Shared modules -->
    <set-configuration-property name="gin.ginjector.modules"
                                value="com.gwtplatform.dispatch.client.gin.DispatchAsyncModule"/>
    <extend-configuration-property name="gin.ginjector.modules"
                                   value="com.gwtplatform.samples.mobile.client.gin.SharedModule"/>
```

For people wishing to introduce presenter specific behavior, it "should" (I'll be confirming this when asking my PR for the mobile sample) work by binding your real implementation to an abstract presenter that contains the proxy. 

Else, a better pattern would be to create a collaborator used by the presenter that depends on the devices and the module bound. As a bonus, your presenter will become easier to maintain and would respect SRP.