Adds the error log handler to the list of supported handlers

Currently there is no support for using the error log handler that `Monolog` provides, using the same configuration that allows syslog, firephp etc. to be set up.

This patch allows a configuration to be set up as follows (YML in this example):

```yml
monolog:
    handlers:
        errlog:
            type:  errorlog
            level: error
            message_type: 0
```

`message_type` is either zero or four and defaults to zero