Fixes Bug 17 - After Window Resize, need to update the height of the grid

Fixes Bug #17

Didn't really see any contribution specification.  This change fixes three things:
1) prevWidth was always the initial width, thus the resize functionality would always get called.
2) Add's the updateHeight() method to refresh the height of the grid element
3) Add's a watch for $elem.width(), which catches some size changes to the dom that window resize does not trigger.  Also handles the visibility case, where width could be 0 (css display:hidden)