<bugfix>Move Vistrail import into load_vistrail method to avoid circular imports.</bugfix>
<bugfix>Move Vistrail import into load_vistrail method to avoid
circular imports.</bugfix>

core/db/io.py:
  - load_vistrail: add core.vistrail.vistrail.Vistrail import, moved
    from header



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