cursorstream - streams2

upgrades cursorstream to a streams2 implementation. some notes:

- drops node < 0.10 support
- you may want to use https://github.com/isaacs/readable-stream if you want to support 0.8. 
- i no longer check the cursor state. you may want to handle that differently.
- you may want to change the tests a little as custom `pause()` and `resume()` methods are no longer used.
- tests don't work for me - they just pause among hundreds of `✔ Should handle uncaught error correctly`s