streams2 write stream

This is the writeable stream component for the [streams2 issue #161](https://github.com/rvagg/node-levelup/issues/106) to go along with [pull #176](https://github.com/rvagg/node-levelup/pull/176). The api is no longer requires listening for a `ready` event before being able to call `.end()`, `.destroy()`, and `.destroySoon()`.

BEFORE (75104e0f4039585ce83fadc2ce155947d97ae60b):

```
$ node stream-bench.js 
--------------------------------------------------------------
Filled source! Took 35280ms, streaming to destination...
Done! Took 36435ms, 103% of batch time
```

AFTER (15c53d7d3487a7994cd5ecbc5f9581d07358f7cb):

```
$ node stream-bench.js 
--------------------------------------------------------------
Filled source! Took 34257ms, streaming to destination...
Done! Took 36344ms, 106% of batch time
```

There is a minor slowdown but it might just be noise.