Fluent API for Vaadlets

I added support for Fluent API into https://github.com/ahoehma/vaadlets.

Now you can do this:

VaadletsBuilder.build(
new Vaadlets().withRootComponent(
    new VerticalLayout()
        .withHeight("100px")
        .withWidth("100px")
        .withStyleName("foobar")
        .withComponents(
                new Button().withCaption("hustensaft")
                            .withAlignment(Alignment.MIDDLE_CENTER)
                            .withExpandRatio(1f))));