Added getStructure() function to the Attachment class.

I think this function would be very useful for places where the existing functions can't provide all of the information. For example, I need to get the content ID of all images from an email, at the moment I'm having to grab the message structure and parse through it while taking educated guesses as to which Attachment ties to which part. 

Getting the Attachment's structure directly would allow for this without any guesswork.