[plt-scheme] new versions of httper, srfi19, uriframe

New versions of a few small PLT-specific libraries.  No major changes
(except the module names for "srfi19" have changed due to unitsig'd
locale); just pushing out whatever's been sitting in CVS.

    http://www.neilvandyke.org/httper/
    http://www.neilvandyke.org/srfi19-plt/
    http://www.neilvandyke.org/uriframe/

BTW, if anyone is actually using UriFrame as a framework, let me know.
Based on what I've learned, I'm leaning towards forking the code to make
a pure-functional URI abstraction and simplify the extension semantics.
(In GC We Trust.)  That might then be a good starting point for a URI SRFI.

#uri"http://www.neilvandyke.org/"