[WO-286] change data model to support html and signed msgs
remove property raw after parsing and properly handle async parsing
remove unnecessary api
rename messageparts to bodyparts
body is per default an empty array if not assigned otherwise later
rename signed property to signedMessage
add mimetype, id, filename to attachment
add fallback for filename
fix meta node handling
Merge pull request #5 from whiteout-io/dev/wo-286

[WO-286] change data model to support html and signed msgs