Fix resizing bug in range_selection tool

Resizing didn't save a mouse-down point. This, combined with that fact that
selections below the `minimum_selection` size get deselected resulted in
strange errors. This also led to exceptions if the event state was set to
"selected" before initial resize event.
Reimplement fix to range_selection resize bug

Reset the `_down_point` on button up and ignore the minimum_selection test when `_down_point` is empty.
Merge pull request #98 from enthought/fix/range-resize

Fix resizing bug in range_selection tool