Deploy to Heroku

This pull request makes this app deployable to Heroku by clicking a ['Deploy to Heroku' Button](https://devcenter.heroku.com/articles/heroku-button) button.

## Demo

[http://vzvu3k6k-xpathfeed.herokuapp.com/](http://vzvu3k6k-xpathfeed.herokuapp.com/)

[![Deploy](https://www.herokucdn.com/deploy/button.png)](https://heroku.com/deploy?template=https://github.com/vzvu3k6k/xpathfeed/tree/heroku)

## Motivation

Feed generation services are often required to sustain for very long, but some of them are eventually closed (e.g. My RSS, なんでもRSS). If we host the service ourselves, we can spare worrying that it may disappear someday.

The 'Deploy to Heroku' button enables us to host this app easily and at low cost.