Added ability to sort columns using local table data

Flexigrid will sort columns using local table data.
- Column TH must specify abbr attribute (used for sort command)
- Optional datatype attribute allows for more accurate sorting.  Allowed
values are number and date.  String is the assumed data type if nothing
is specified.

View usage example on jsFiddle: http://jsfiddle.net/6WBSF/