Default to disk storage

The readme states

> In case you omit the options object, the file will be renamed and uploaded to the temporary directory of the system.

However looking at the code if the options is undefined, it passes in an empty object.
Then if storage and dest are undefined it uses memory storage.

I got the behavior I expected the library to have by default with

``` javascript
var multer = require('multer')
var tmp = require('tmp');
var upload = multer({dest: tmp.dirSync().name});
```

I would suggest either updating the docs or updating the implementation to be consistent with the docs. I prefer the latter.