Add FromCenterHeaderTransformer and make ListView sample use it.

One possible solution for chrisbanes/ActionBar-PullToRefresh#12.

This is the first implementation that came to mind, but there are still a few formalities that might need sorting:

* Should this replace DefaultHeaderTransformer?
* FromCenterHeaderTransformer depends on the fact that the ProgressBar resides in a LinearLayout (in order to make use of the ProgressBar's layout gravity). Would this be acceptable considering this considition is met with the default header layout?
* Is there a better name than FromCenterHeaderTransformer? (In the case that this transformer becomes default I guess it can just be called DefaultHeaderTransformer and we can move on.)