layer comp poster support

- update when the document changes
- allows empty spec names like “default 100%, 200% retina/@2x
- uses getDocumentPixmap API for rendering
- updates npm-shrinkwrap
- renders SVG layer comps when possible
- lint and use Q properly
Minor spacing cleanups
Merge pull request #260 from adobe-photoshop/jhatwich/layerComps+size

jhatwich/layer comps+size