Allow the datagrid body to scroll without scrolling the header.
This does some heavy surgery to Datagrid's handling of the table header.

Tables aren't able to scroll only their body. All thead tags will scroll

along with it. My goal was to allow the body to be scrolled without the

header, in order to create a nicer experience with large numbers of

rows.
To make this work, we create two tables: One for the header, one for the

body. We pull out a copy of the header from the original body and place

it in the header table. We then do some work to synchronize all the

widths and to ensure that we don't lose their natural widths.
The details are in the comments in the code, but the end result is that

the table acts as it did across all major browsers, with the difference

being that the body is scrollable.
Now to make it scrollable, a height must be assigned to the datagrid

element, and resizeToFit must be called on resize (after appropriately

sizing the datagrid). The datagrid will then do the rest.

Tested with my accompanying Dashboard changes, and with the more plain

datagrids on the Groups and All Review Requests pages.
For the dashboard, the body scrolled without disrupting the header.
For the others, they behaved as before. Scrolling was handled by the

browser.
I tested reordering columns, adding/deleting columns, and resizing the

window.
This was tested in Chrome, Firefox, IE10 and IE 7, 8 and 9 compatibility

modes through IE10.