improved handling of empty adapter. Still a lot to do on this though
fixed the ordering of method calls in set adapter
made changes to ondataset change to handle an empty dataset
fixes issue #11 and fixes #12

- Now fully supports null adapters as well as adapters with a count of 0.
- Added setEmptyView() method that shows a view when the dataset is empty or adapter is null. This works like ListView's method with the same name.
update readme to reflect recent changes