FIX: Tab/Shift/Alt/Ctrl/Meta keys mark radio buttons as checked

When a radio button is selected and Tab/Shift/Alt/Ctrl/Windows/Cmd keys fire the 'keyup' event, the focused control gets selected for no apparent reason. 

From a user perspective, this behavior is a bit confusing, because jumping from a textbox to a radio button marks de latter as selected.

https://github.com/fronteed/iCheck/issues/18 Addressed something similar

Thanks for your work on iCheck!