notmodified: add NotModifiedError() class
ftp: better ftp impl, not with NotModified support
notfound: add NotFoundError class
ftp: handle the "file not found" scenario
data: add NotModifierError() handling logic

Taking an SHA1 shasum of the URI string before processing it,
to potentially avoid the creation of the Buffer from the "data:" URI string.
notfound: fix jsdoc description
data: fix debug() function name
README++