Add support for row CSS style

Implement CSS style for each row in grid.
This is very useful to dynamically apply a row style, depending on the data.
See rowStyle sample provided.
Add a sample for the rowStyle.
For invalid dataIndex, getColumn() returns the last column in grid

If no column is available for the 'dataIndex' argument, then the
'getColumn()' function returns the last column in the grid. It should
return 'null' instead.
Re-formatting
Update to work with 2.1.0 final
Define 'column' variable before the for loop like it was
Minimize the calls for Ext.bind

Instead of calling the Ext.bind each time a row template is built,
create the function once and save its reference.
Issue #31

Ensure the item template gets applied when the grid structure changes:
toggle columns.
Synch with master
Merge pull request #27 from adrianbadea/master

Add support for row CSS style