Caches column rects

Caching column rects to improve keyboard scrolling performance with large numbers of columns.

Signed-off-by: Brendan Duddridge <brendand@gmail.com>
Merge branch 'master' of https://github.com/brendand/mbtablegrid
Merge branch 'master' of https://github.com/brendand/mbtablegrid
Merge branch 'master' of https://github.com/brendand/mbtablegrid
Support for returning different NSCell subclasses to construct the grid

Signed-off-by: Brendan Duddridge <brendand@gmail.com>