Add support for enter, exit, update, hover props
In transforms, check for .update props
Update tests for .update props
Add utils_props file
Add tests for prop_sets
Small test cleanup
Update test spec results

Props like x and fill have been renamed x.update and fill.update, resulting
in different data names.
Add drop_props function
Add hover demos
Set default size for mark_symbol
Merge branch 'hover'