Attachment filename support

This PR adds support to look up any related attachments (photos, contacts, map pins, etc) to a message, and to include a path to the file in the final result under a `filename` key. 

This is the first Python I've written – I can't imagine this is great code. I'm just sticking it up here so somebody else might get some mileage out of it. The `replace` on `ORIG_DB` is particularly gross, and I'm sure I've done other things wrong as well. 

Sample response:

    {
      "date": "2014-08-04 14:25:33", 
      "filename": "/Users/Andrew/Library/Application Support/MobileSync/Backup/[...]/69153a839cda555c05a8f25aed66c9817ba06822", 
      "from": "[...]", 
      "text": "￼I thought you might appreciate this lovely photo.", 
      "to": "Me"
    },  

The `filename` points to a path on the local file system within the iOS backup that is a JPEG/PNG/VCard file.