core for dfsplice object is now stored in dictionary, only path and cache environment are added at runtime by the splice function

- Minor changes in error messages
- Row names file is not compulsory anymore
- Column name, type, summary and attributes are stored
store original attributes as attributes and not inside an attribute

so that semantics of "comment" are maintained
restrict length of summary to 7 items
don't store levels
improve str.dfunbind (and print.dfunbind)
Merge branch 'master' into 7-dict
can't store column attributes as object attributes
NEWS
bugfix
isplice also looks in inst/extdata, for devtools's local installation
bugfix: print comments
Merge pull request #8 from krlmlr/7-dict

core for dfsplice object is now stored in dictionary, only path and cache environment are added at runtime by the splice function