Add attribute for setting main.cf variable 'header_size_limit'

I'm using Postfix to relay mail to SendGrid.  In SendGrid docs for this they specify configuring main.cf with:

```
header_size_limit = 4096000
```

http://sendgrid.com/docs/Integrate/Mail_Servers/postfix.html

Nice to be able to do it from my Postfix Chef cookbook so I added the attribute and corresponding lines in the main.cf template.  Probably some other folks out there integrating with SendGrid who would like this too.