Fixed bug preventing positioning of nodes within Backdrops.

The collision group mechanism may seem a little more general than is needed for this particular fix, but I believe it has uses down the line, in particular for laying out individual islands of nodes separately before aligning them as required by #155.

Fixes #769.
Improved NodeGraph.frame( extend=True ) behaviour.

It was enforcing a maximum zoom level, which is essential for the extend=False case, but annoying in the extend=True case. If a user has chosen to zoom in a long way, then we shouldn't change that unless it's totally necessary to show the nodes.
Merge pull request #770 from johnhaddon/layoutFix

Fixed bug preventing positioning of nodes within Backdrops.