not exact grouping / grouping by intervals of values

Hello,

after writing [the answer](http://stackoverflow.com/a/16152739/315935) I examined more exact the possibilities extending of jqGrid grouping so that not only exact values can be used to build groups. I think that I found very simple and flexible enough way which

 1. support of "not exact" grouping by introducing `isInTheSameGroup` array of callbacks in `groupingView`.
 2. more flexibility in constructing of `displayField` by introducing of `formatDisplayField` array of callbacks in `groupingView`.

The results of the suggested modifications one can see on [the demo](http://www.ok-soft-gmbh.com/jqGrid/NotExactGrouping1.htm) which displays the results like the following

<img src="http://www.ok-soft-gmbh.com/jqGrid/NotExactGrouping1.png"/>

One can see that the grouping on the first level will be done in case-insensitive way and that the values will be displayed in low case in the grouping header.

One can see that the grouping on the second level will be done by *intervals* of values (by year and month). The  grouping header display also no day of the grouping element.

I tried to make the changes to make full compatibility with existing grids.

I hope that such changes will be helpfully for other users of jqGrid.

Best regards
Oleg