Add component.json.

This enable underscore.js to be used as a component as per
https://github.com/component/component#readme
Automatically build component.json from package.json.

Do `rake component_json` to update component.json to match package.json
whenever package.json is changed.
Automatically build component.json on every release.

Runs `rake component_json` automatically whenever `rake build` is run.
This way, component.json is up to date with package.json at every release,
and there is no manual step required to manage the component.json file.
Add comment about not editing component.json automatically.
TEMPORARILY change repo to hugojosefson/underscore.

This is to enable use of underscore.js as a component from
hugojosefson/underscore until the commits up to this one
have been accepted into upstream documentcloud/underscore.

THIS COMMIT WILL BE REBASED AND/OR REMOVED IN THE FUTURE.