expose dowloadFromInfo and drop some dependencies

This pull request:

 * retains backward compatibility
 * removes request caching. Instead you are supposed to use `getInfo` followed by `downloadFromInfo`.
 * drops dependency on eventvat
 * drops dependency on streamify, uses built-in `PassThrough` stream instead.

All tests pass.
Even if it seems roundabout, I still think consistent APIs are more favorable. And from what I've seen, users seem to prefer that something already returns a stream to work with, even if it's not the "real" stream. Rather than wait for it in a callback.