idea for a simpler API / drop some dependencies

Here is an idea for a simpler API. This breaks backward compatibility. Here are the features:

 * does not depend on eventvat. This eliminates a dependency and makes the memory footprint of ytdl more predictable. Yet it still avoids duplicate requests by adding the `downloadFromInfo` function.
 * does not depend on streamify. Switches to a simpler callback interface.

All tests pass.