Merge pull request #632 from guimeira/unstable

Added the wheelSensitivity initialisation option
Fix concentric layout example in src #633
bug fixes; add dagre layout; cola layout seems coupled with d3 unfortunately
new arrows; mid arrows (bezier+haystack); +docs

Triangle-tee arrowhead #620
Pointy arrowhead style #619
Mid arrow styles + rendering support #557
is.js uses 'instanceof' instead of Array.isArray #641
Little bug in css() function #638
cxttapstart on everything but nodes and edges #637