Use `he` for robust HTML entity encoding/decoding

_ent_ has [several open issues](https://github.com/substack/node-ent/issues/created_by/mathiasbynens?state=open) that are not present in [_he_](http://mths.be/he).

I’ve also updated the URLs in the `package.json` as they didn’t work anymore.
This avoids the following warning message:

```
$ npm install
npm WARN package.json node-twilio@0.1.1 bugs['web'] should probably be bugs['url'].
```