<bugfix>API: return result after execution</bugfix>
<bugfix>API: return result after execution</bugfix>

core/api.py:
- VisTrailsAPI.execute: Return the result of the execution

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