Added attachments

Added two lines to Message class to support attachments and an Attachment class to represent attachment objects.  Attachments to an email are stored in an array (message.attachments).  Attachment objects have name and size (kilobytes) properties, and their associated files can be saved to disk using the save() function, which optionally takes a path to save the file to.  I updated the README to show capabilities, though you very well might want to represent the information differently.  Play around with the changes, pull them if they're up to snuff, let me know if they're not.  I'm working on a gmail-based Twilio clone (twilio.com if you're not familiar) and I'm using your library but I needed attachments capability for MMS support so I figured I would just see if you would pull my changes.  Thanks!