add --dev install option for devDependencies

For #80

You can now do `bower install --dev` or `bower install -d` and bower will add packages from `devDependencies` from your project's JSON.

Has basic test. It's fairly simple, extending `manager.json.dependencies` with `manager.json.devDependencies`

---

This does not solve for resolving endpoint devDependencies, i.e.

``` bash
bower install package-name --dev
```