executable_file_exists(): new function.
executable_file_exists(): new function.

VisTrails/VisTrails,celiafish/VisTrails,Nikea/VisTrails,minesense/VisTrails,Nikea/VisTrails,hjanime/VisTrails,minesense/VisTrails,Nikea/VisTrails,minesense/VisTrails,celiafish/VisTrails,VisTrails/VisTrails,celiafish/VisTrails,hjanime/VisTrails,Nikea/VisTrails,minesense/VisTrails,celiafish/VisTrails,VisTrails/VisTrails,minesense/VisTrails,hjanime/VisTrails,hjanime/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails