Add simple logging

This will write log files one level above the document root to `error.log`. It also replaces the default error handler with [Whoops](http://filp.github.io/whoops/demo/) so it’s prettier.

We can use any handler that’s available with Monolog 1.7 (they’re listed on [this page](https://github.com/Seldaek/monolog/tree/1.7.0)), any suggestions for something more useful we could do with live sites? I’ve a feeling error logs will be neglected!