Updated mail library to use SwiftMailer

Lately I have been having numerous encoding issues with the mail library. Rather than continually try to fix these issues I decided to just update the mail library to use the SwiftMailer library. 

By using SwiftMailer Opencart will no longer have to maintain a mail library and can therefore leave this task to another project. The mail.php will be significantly smaller.

Note: I have added SwiftMailer using composer but you could easily just drop the SwiftMailer source into a sub-folder inside the library folder and then require it manually from the mail.php class.