Gridcommands

This is a bug fix for bug issue #11 (Custom commands in grid not functional).

The proposed solution is to iterate over the columns collection and convert any custom commands from their string value to a function reference by using eval(). 

I know that using eval is generally frowned upon, but this seems to be the only solution due to the fact that the grid expects the data in the columns array to be proper JSON and will not accept anything else. Someone with better knowledge of the grid's internals may be able to propose a better solution, but this seems to work well.