Extend meta data on Comment

Extend Comment with:
       - ByLine (author) info
       - Proper Time type
       - ExternalID
       - Source info (Via)
       - New creation method (CommentAttr) that enables setting meta-data
ID -> Id
simplify time unmarshalling
only have one Comment creation method
Merge pull request #3 from stengaard/extend-comments

Extend meta data on Comment