Add npm metadata: files, homepage, bugs, and repository.

Following the guidance of the [package.json](https://www.npmjs.org/doc/files/package.json.html) docs, I've added a few keys to improve package discoverability and tarball size.

* `files`: When employed as an npm dependency, we only want the lib code. There's currently a lot of extra stuff included in the package downloaded from npm (docs, test, dev metadata files, "callbackhell.js", etc).
* `repository`: Removes "No repository field" warning when installing.
* `homepage`: Makes `npm home highland` and `npm docs highland` open the correct page (highlandjs.org).
* `bugs`: Makes `npm bugs highland` open the appropriate page (GitHub Issues)

`homepage`, `bugs`, and `repository` also add useful links to the header of highland's [npmjs.org homepage](https://www.npmjs.org/package/highland).