Add API for showing multiple Showcases

[amlcurran]

This seems to be a commonly requested feature.

My current thinking on this API is usage along the lines of:

sv = new ShowcaseView.Builder(this)
                .withMaterialShowcase()
                .setTarget(target)
                .setContentTitle("Item one")
                .setContentText("Some description")
sv.thenShow()
    .withMaterialShowcase()
    .setTarget(target2)
    .setContentTitle("Item two")
    .setContentText("Some description")
    .build();