feat(diagram-export-image): export diagram as JPG image

See #8.

Bug:
- Does not properly export connections

See
https://cloud.githubusercontent.com/assets/1885333/5482968/8fa18588-8640-11e4-8c1b-f6bc9e87212b.png
feat(diagram-export-image): complete support for Diagram to Image

Closes #8.

Uses CanVG to fix previous issue that Connections and Endpoints were
excluded from final rendered image.
Merge branch 'master' of github.com:mozilla/seasponge