Browser detection: Add Atom shell

I'm developing a Atom plugin that will add a paper.js repl. Eg. you can edit your paper code in one panel and see the visual result in another panel.

Atom is running in an unsual mix of both node and browser environment at the same time. Currently paper.browser detects it as browser.chrome. This enables some feature that aren't helpful in atom, so with this commit I added another browser to paper.browser.