Heroku support

<del>Waiting on merge of (forthcoming) CakePHP support in the CHH/heroku-buildpack-php#73 buildpack.</dev>

### Summary

- Allows users to configure their database using a `DATABASE_URL` environment variable
- Adds support for configuring the cache using <del>`MEMCACHED_URL`</del> and `REDIS_URL`. Memcache requires a Memcached engine (not in 2.4.x) with SASL support.
- Adds the ability to set the `LOG_PATH` env var, which is useful for heroku to centralize logs. Defaults to old `LOGS` value.
- Sets the db connection encoding to `utf8`.
- Adds some helper methods for dealing with environment variables
- Adds some information on configuring your application for heroku support

### Test application

[This app](http://cakephp-test.herokuapp.com/) has the buildpack in use (no customizations other than whats in this PR, plus a few env vars): http://cakephp-test.herokuapp.com/

Seems strong so far.

### Todo

- [x] Test pretty urls
- [x] Unify with the nginx config we have in the book
- [x] Ensure <del>Memcached</del> and Redis support work as desired.