Allow empty bundles but mark them as "empty":true in the output tree.

This allows consumers of the output tree to skip empty bundles if they desire, although including them would be relatively harmless.

Closes #17.
Move log.{out,err} into lib/util.js.

Since the colors are defined in util.js, it makes more sense to fold the color option into the log functions as an optional second parameter.
Warn (but do not die) when files cannot be watched.
Handle false positive dependencies more gracefully.

Closes #18.
Bump NPM version to 0.4.1 in preparation for publishing.

Although the recent changes to indicate empty bundles were intended to be backwards-compatible, any change to the output format deserve a minor version bump.