Switch to Closure Compiler

[flack]

From #29 (comment) :

If I compile the current jquery.jqGrid.src.js from my my repository with respect of Closure Compiler I get jquery.jqGrid.min.js of the size 275 KB (282.532 bytes), but if I compile the same file using Uglify I get minimized file of the size 277 KB (284.619 bytes). So the usage of Closure Compiler seems be the best choice if one take in the consideration the final results.

https://github.com/thanpolas/grunt-closure-tools

which seems to provide all the configuration we need.