Continue refactoring

This is a follow-up of juriansluiman/SlmMail#2. The current state is **done** but I'd like to keep ppl up2date with my changes.

Done:

1. Check API calls for ElasticMail, Postage and Postmark (just a double-check to be sure because it's not testable (yet))
2. Repair SendGrid so basic email sending works
3. Add some more info in the exception message
4. SendGrid completion (esp. error handling must be tested)
5. SendGrid documentation
6. Attachments (like fetching text & html from the MimeBody, you can do the same with attachments)
7. Error handling in ElasticEmail (their API kinda sucks, but you can get error states out of it)
8. Php 5.3 support (removal of traits)

What needs to be done:
1. Test Mandrill service with a) tags b) options c) attachments