Fixes to isSelected

Make isSelected() deal more correctly with falsy ids, and also allow it to return the correct value when selectAll has been issued, but the user has not scrolled through all the data in an onDemand list.
Adjusting ColumnHider menu height dynamically

In cases with several columns and a relatively short grid, the ColumnHider
menu could exceed the bounds of the grid's viewport, but could not be
scrolled, resulting in the latter columns being inaccessible.

These changes make some small CSS modifications to allow for scrolling within
the menu if it's taller than the grid, while also adjusting its size to never
be greater than the grid itself. Given that it's currently positioned
absolutely, the height is being determined and set in code, rather than via
CSS.
Adding test
Merge branch 'columnhider-menuheight' of git://github.com/brianarn/dgrid into columnhider-menuheight
Cleanup for #311
Fix #313: rename class used for csstransforms3d has test
Make has-css3 reuse has.js's DOM element instead of its own
Update CHANGES.md
Fix #312: allow HTML in loadingMessage, consistent with noDataMessage
Update CHANGES.md RE loadingMessage
Add height/width to imglist rows in OnDemand_tile test
Update README.md

Switch recommended Dojo version to 1.8.1
Update version number in CHANGES.md
Further event normalization fixes
selector: fix fallback when column.label isn't set; refs #324
Make column definition objects optional to OOTB plugins; refs #324
Update CHANGES.md for selector fixes
Added horizontal scrolling to ColumnSets using the mouse.
Ensure that when we adjust row indices (after insertRow removes a node), that we only increment from a real row, fixes #318
Set editor value back to original value if dgrid-datachange is prevented.