Fix bug in 'reset' seen when number of rows changed.
Add DataSource<T>.clear() method.
Fix bug when window offset changed. Expand unit test.
Grid adjusts window size to fit parent element. Closes #3.

Required using the 'scroll' change event in more places to avoid infinite
loops of event handlers when performing batch operations. Now, the scroll
event is fired once at the end of the batch.
f Grid adjusts window size to fit parent