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.
Added TableTools. Added export for CSV and PDF to measurement table
Fix export widget - export NetworkX json graph

Uses the set of neurons in the selection table, and now reuses
the same, fast backend-function that is used for the graph widget.
Additional comment to store graphml file
Retrieve number of reviewed nodes in export
Confidence threshold and bandwith as optional parameter
Added reviewed count and N-order retrieval
Replace GET by POST requests
Enable creation of simultaneous, independent connectivity widgets
Fixed error in authentication.py: turns out that a list is larger than zero
instead of failing with an Exception.

Literally: in python, a list containing one tuple evals to larger than zero:

>> [('a',)][0] > 0
True

... which makes for on surface incomprehensible errors when the tuple contains a zero:

>> [(0,)][0] > 0
True

One of course has to use two sets of indexing brackets:

>> [(0,)][0][0] > 0
False
Make split_skeleton and _join_skeleton use can_edit_or_fail
which takes into account the user's groups.

While at it, simplified _join_skeleton a little.

Thanks to Matt Berck for pointing this out.
Merge branch 'master' of github.com:acardona/CATMAID