Adds a menu based GUI for updating R within Rgui.
add.installr.GUI()
Returns invisible TRUE/FALSE if menus were added or not.
This function is used during .onLoad to load the menus for the installr package in Rgui.
My thanks goes to Yihui and Dason, for the idea and help with implementation. See also: https://stackoverflow.com/questions/15250487/how-to-add-a-menu-item-to-rgui/
Tal Galili, Dason
if (FALSE) { add.installr.GUI() }