Added a custom serialize method for dhtmlxgrid.

DHTMLX libs GPL version 3.6 do not support .serialize(). It is 
available only in the PRO version.
Refer - http://docs.dhtmlx.com/api__dhtmlxgrid_serialize.html
Therefore a simple custom serialize method has been added to support the
missing serialize functionality.

https://bugzilla.redhat.com/show_bug.cgi?id=1101250
@AparnaKarve Haven't tried this out yet, but let's stick with putting an miq prefix on the function like the rest in this module, so miqDhtmlxgridSerialize . . . thx, Dan