Fix for Nested Attachments (Issue #13)

[dzastera]

Updated attachment processing to handle nested attachments and updated the regex on msg_fields to be more robust with grep.

Nested attachments contain the string "__attach_version1.0" more than once, which was not handled uniquely with grep resulting in all nested attachment information being written out when saving, rather than just the attachment information.

This fix addresses the uniqueness issue by using more of the key to keep the attachments distinct. The regex on the 'msg_fields' items have been modified to include the terminal 4 hex digits in each value. The addition of an end-of-line anchor helps assure that the msg_fields are not matched mid-key - an issue with nested attachments.