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.
Using 1 PassThrough stream seems worth it because it offers users an alternative API, even if it's slightly less efficient. Using 2 would offer the same API with the slightly less efficiency of an extra stream.