Added enter keypress for overflow menu. Added check for tabindex so NaN is not inserted into HTML.
Added enter keypress for overflow menu. Added check for tabindex so NaN is not inserted into HTML.

zkoehne/Office-UI-Fabric,OfficeDev/Office-UI-Fabric,waldekmastykarz/Office-UI-Fabric,zkoehne/Office-UI-Fabric,waldekmastykarz/Office-UI-Fabric,OfficeDev/Office-UI-Fabric