Migrate to pipe. Dropped rotation options.

Fixes #28 

There are some things that may seem odd about this code, (`file` vs `directory` options) but there are plans to add the time rotation back in and I left some of the logic in there to make that easier.

- TODO add more validation on the options. https://github.com/hapijs/good-file/issues/33
Per https://github.com/hapijs/good-file/blob/master/test/index.js#L70-L93, if I didn't explicitly call `.end` that would evaluate to `true`. I am also setting `end` to `false` when creating the write stream which I think trumps passing in `null` to end the stream. I think?

What should the order of operations be in this tear-down? I've heard a few different things from different people and I'm getting a little confused at this point. I think it should be:

```js
this._readableStream.push(null);
this._readableStream.unpipe();
this._writeStream.end();
```