tests: Improve error message on image comparison fail
Add initial support for NoFlo runtime w/noflo-canvas

Adds a trivial demo to the regression checks. Currently requires some hacks:
* "npm install noflo-core noflo-canvas" manually
* CreateCanvas with hardcoded dimensions in the graph
* "input" url needs to be specified, even if not used (see tests for example)
Add noflo-core and noflo-canvas to dependencies

Required by recently added noflo-canvas graph supporT
Travis: Install libgif dependency
server: Make 'input' attribute optional

Avoids having to download a dummy file for purely generative graphics
server: Automatically create canvas, pass height/width for noflo-canvas graphs

All graphs for noflo-canvas now accept height+width parameters on HTTP,
and must expose one "canvas" inport to accept the canvas instance they
should draw onto.
doc: Update README about noflo-canvas graphs