Add RTL support

Modified Slick.Grid to allow RTL support.
Modifications were brought from meyogi's fork (https://github.com/meyogi/SlickGrid) and updated to 2.1.
There might still be a few issues (I noticed re-ordering of columns to be an issue - might be an inherit problem in jquery.sortable).
I've also added an example to test some of the functionality I noticed might be an issue (re-order, re-sizing, sub-header...).