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.