Implemented risk measurement for edges in the connectivity graph between neurons.

The javascript side needs work.
graph.py: do not label branch nodes when minifying an arbor by synapse clustering.
graph.py: do not compute risk for autapses; rather, label them as so.
Better docs for a function in tree_util.py
graph widget: checkbox toggles for names and to cut names at the first semicolon.
graph.py: compute edge risk only when not minifying arbors.
Make the middleware report the full stack trace of any exception.
graph.py: label branch nodes appropriately in minified arbors.
graph widget: simplify and improve.

 * start off with a force-directed layout.
 * render branch nodes smaller in minified arbors
 * simplify GUI: no more "update" button, selecting the layout triggers re-layout.
 * acknowledge state: if short labels, or if no labels.
Show more reasonable ID in a CICI link's text representation

With respect to a classification root node, class_instance_b of a
ClassInstanceClassInstance (CICI) link is the actual root class instance
of a classification graph. Its class_instance_a would refer to the
linked project. Since there can be many CICI links to the actual root,
but from different projects, it is makes sense to not show the link's
ID, but the root's.
Importer: change style to fit more into admin layout

This commit also changes the user and group permission control elements.
Now the FilterSelectMultiple class is used there, which is in use on
other places in the Djange admin. This increases usability for multiple
selections on large lists.
Importer: allow manual selection of classification graphs to link

If a user chooses to link classification graphs on the first page, (s)he
now able to not only linke the suggested classification graphs, but also
any graph wanted.
Importer: increase width of classification graph control
Merge branch 'importer-update'
Update pgmagick requirement for Mac OS

Installation failed on Mac OS with pgmagick 0.5.5 (libboost library
could not be found because of an error in the install script). This was
fixed in 0.5.7
(https://bitbucket.org/hhatto/pgmagick/commits/977a2d1862553198d347464bb
7ca2ea7efcf6090)
Add ProjectStackInline to ProjectAdmin

ProjectAdmin now shows associated Stacks. Also renamed ProjectInline to
ProjectStackInline to better reflect the model name.
Merge remote branch 'felix/small-fixes'
Fix problems with overlays in stack window

If the slider for an overlay is set to 0 (i.e. smaller than 0.02 to
be precise) - the tilelayer is removed from the DOM and kept only
as a reference. Tiles are then not fetched. In the Django admin
interface, set the default opacity to 1 (or a value in the range 0
to 100) to isolate the tile layer by default when loading the stack.
This allows now to have many overlays attached to a stack, but only
fetch the tiles for the overlay you turned up the opacity value.
To avoid confusion, rename OverviewLayer to TilelayerControl.

Readd the instantiation of the OverviewLayer only for the 'TileLayer'
layer, not for overlay layers, to enable fetching of the small thumbnail
images in the bottom-right map.
Forgotten renaming of tilelayercontrol