Moved rect height dropzone to RHS to prevent overlap with vertical group by dropzone on LHS of layer.
Switched default mark type to cross since trifacta/vega#118 is fixed
Broke Vis.parse into Vis.spec & Vis.render; Vis.importVega renamed to Vis.parse
Extracted vis.reset
Fixing error with pipelineNames when importing Vis
Popping one level of nesting for importVega
We need checkExtents for dropzone interaction, we just don't need it called in vdeProperty.
Incorporating trifacta/vega#203, a fix for trifacta/vega#111 (drag/drops on marks could select group)