Utilize htmlentities gem for decoding/encoding

HTML entity decoding and encoding is hard. There are [so many](http://www.w3.org/TR/WD-html40-970708/sgml/entities.html) different symbols. Also HTML entities can be presented as named strings (`&quot;`) and numerical (`&#34;`).

It was nice trying to handcraft decoding/encoding of these, but it appears that it would be much better to use [standalone](https://github.com/threedaymonk/htmlentities) library, which handles that complexity for us.