refactor: replace callback-style API with Promise-style API

[malept]

BREAKING CHANGE: drops callback-style support in favor of a Promises-based API. If you still need callback-style support, see util.callbackify.

Additional changes:

• Refactor the code so that's it's encapsulated in a class, instead of in one monolithic function (with nested functions)
• More modern Node refactors
• Replace tape with ava
• Replace concat-stream with get-stream
• The code base should have full test coverage now

Fixes #81
Closes #46