Add bower.json

Prism is already registered with bower. This bower.json file just ignores the website-bits of the repo.

Running `bower install prism#gh-pages` will install these files:

```shell
.
├── components/
├── plugins/
├── bower.json
├── LICENSE
├── prism-coy.css
├── prism.css
├── prism-dark.css
├── prism-funky.css
├── prism.js
├── prism-okaidia.css
├── prism-tomorrow.css
├── prism-twilight.css
└── README.md
```

Related issue: #52
prism.css doesn't appear to be at the root level anymore.