npm: remove unused devDependency
Add jslint, but don't invoke it yet
jslint background.js

Although the diff shows the whole file changed, the only changes were
a whitespace here and there, but a global indent ruins it all.
Add background.js to Makefile
Don't specify a task for grunt, use default
Refactor Makefile to handle dependencies better

The main change is not to have 'test' as a separate target, instead call
grunt as a part of zip-up target.