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.
If you're concerned about efficiency about using multiple `PassThrough` streams, then perhaps your users might be concerned as well, and you should expose a callback interface that returns the real stream? But you've already said that you don't want to do that.