New Feature: Option to specify the watch event type

This is useful for generating manifests of types of files, you would only need to generate the list on an add or delete, not on a change.

No tests included yet, I ran the existing ones before I changed the source and they were failing so I thought I'd wait to see what you guys think before I dove into that. Looks like the travis build is failing on `npm install`ing `uglifyjs` under node v0.10.

Note: I chose not to use Gaze's `.on(type)` because it does not seem to support multiple events.