- changed "missing" to colander.drop instead of None or
  empty lists so the resulting appstruct can be passed back
  into schema.serialize() without issue
  (Ex. Integer types caused colander.Invalid as None is not
  a valid integer value)
- likewise, don't add None values to dictionaries returned
  by dictify() so that can be used as an appstruct
- modified detection of autoincrement fields so they can
  be properly set to missing=colander.drop (wasn't detecting
  ForeignKey constraints)
Merge pull request #42 from tisdall/master

fixed issues with None-like values and autoincrement fields