Disable Guzzle exception for error responses

Our HTTP client is expected to return the response for 4xx and 5xx, not to throw an exception
Merge pull request #35 from stof/patch-1

Disable Guzzle exception for error responses
Update Identity.php
Moved test Postmark class to separate file
Fixed invalid Postmark tests
Updated MailGun tests to use base tests class
Added Attachment classes
Passed message attachments to adapters
Processed attachments for mailers
Added tests for attachments
PHP 5.3 compatibility fixes
Removed isset usage
Corrected documentation for message attachment interface
Coding standards compatibility
Removed callback usage when processing attachments
Renamed message attachments container interface
Fixed incorrect code coverage tags
Added support for file uploads with Buzz
Improved attachment tests
PHP5.3 test compatibility

Hopefully the last one