Use strict mode

Enables use of `node --use_strict` by packages that rely on this.

`mkdirp` is also guilty of this (one of this package's dependencies). I've [submitted a PR to them](https://github.com/substack/node-mkdirp/pull/36), but the maintainer has been unresponsive (see [the issue mentioning it being broken in strict mode](https://github.com/substack/node-mkdirp/issues/32)), so it may not get accepted soon, if ever. In the meantime, you could link directly to my branch in your `package.json`, if you would like. Example:

```
"mkdirp": "git://github.com/bfrohs/node-mkdirp.git#strictMode"
```