addition of interface for consistency with base64 module.  Namely,

encodestring(), decodestring(): New functions which accept a string
object and return a string object.  They just wrap the string in
StringIOs and pass them to the encode() and decode() methods
respectively.  encodestring() accepts a default argument of quotetabs,
defaulting to zero, which is passed on straight through to encode().

encode(): Fix the bug where an extra newline would always be added to
the output, which prevented an idempotent roundtrip through
encode->decode.  Now, if the source string doesn't end in a newline,
then the result string won't end in a newline.

Also, extend the quotetabs argument semantics to include quoting
embedded strings, which is also optional according to the RFC.

test() -> main()

"from quopri import *" also imports encodestring() and decodestring().