remove WriteStream, replace in tests with level-ws

PR is against 1.0.0-wip / #289 / @kesla 

I've removed write-stream.js but have left the tests intact by using level-ws but it'll depend on 1.0.0 if & when Level/level-ws#2 is merged.

The only part missing is the fstream stuff for functional tests. I'll work on that next.