Fixes #15. Allows a specified directory name to be used

* Backward compatible with the old scheme that uses the name
* Added a test
* Updated readme
How about providing a custom generator here?

e.g.

```
new tmp.Dir('name', { generator: function() {} })
```
Or if you don't want to use a generator at all - 

```
new tmp.Dir('name', { generator: false })
```