Add a lasso for selecting spots

[eriksjolund]

Add a lasso for selecting spots to the
from_layout viewer

One way to do it is by using Paper.js together with
https://github.com/eriksjolund/OpenSeadragonPaperjsOverlay

The function
http://paperjs.org/reference/path/#hittest-point
would then be used for determining if a spot is inside the lasso.

It could be done in a similar way as it is done in:
http://eriksjolund.github.io/osd-spot-viewer/adjust_spots/adjust_spots.html
but here only the lasso would be painted by Paper.js not the circles.

By painting the circles directly on the tiles, the circles are shown without any wobbling effects.
Obects painted by OpenSeadragon overlays (e.g. OpenSeadragonPaperjsOverlay) tend
to have some wobbling effects when zooming in or out.