Add HTML email formatter

This Formatter formats incoming records into a HTML table to create a readable email logging notification.

The email result is something like this:

![Example](https://f.cloud.github.com/assets/1071148/1577936/504cf4ca-5172-11e3-8044-3db06596abbe.png)


This have one requirement, your email must be sent with **content_type: text/html**

Example in Symfony 2:
```yaml
monolog:
    handlers:
        #other handlers...
        swift:
            type:       swift_mailer
            content_type: text/html
            #your configuration...
```