Memory leak on ui-grid-menu showMenu

Hi,

On ui-grid-menu.js, there are 2 methods showMenu and hideMenu that are added to scope but also are added to some self variable that happens to be the ``window` object.

The problem is that in a spa app, navigating away from the list, on the window object that is not going to be destroyed, the 2 functions will remain with the scope as context variable, so all the grid objects, columns, rows will be carried through the rest of the journey.

The 2 functions seem to be used only through scope, so no need for them on the ``window` ... at least to the point I got now.