Add keys :: String -> ForeignParser [String]

This allows reading keys of an object by key:

    data Project = Project [String]

    instance readForeignProject :: ReadForeign Project where
      read = do
        dependencies <- keys "dependencies"
        return $ Project dependencies

    parseJSON "{\\"dependencies\\": {\\"purescript-react\\": \\"*\\"}}"
      :: Either String Project
readKeysImpl now checks for null and undefined of its arg.
readPropImpl now checks for null and undefined of its arg.
Merge pull request #6 from andreypopp/master

Add keys :: String -> ForeignParser [String]