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.
Hmm, do we really want to provide two differen APIs? I know it wouldn't be as intuitive to have `downloadFromInfo()` return a stream too because this function calls `getInfo()` but still needs to return a stream.

Maybe we can have another function that's exported instead of `downloadFromInfo()`, and it creates a stream and then calls `downloadFromInfo()` with the `info` from the user. What do you think?