Added methods for setting Mailgun settings:
- Enable/Disable DKIM Signatures
- Enable/Disable Tracking
- Enable/Disable Click tracking
- Enable/Disable Open Tracking

This is a follow-up for #19 (@GrandadEvans)
Merge pull request #24 from Sepiksu/master

Updated to Laravel 4.2
Merge branch 'master' of https://github.com/Bogardo/Mailgun
Updated Readme to include docs for newly added features and the installation difference for Laravel 4.2