Add support for uncompressing to iovecs (scatter I/O).
Windows does not have struct iovec defined anywhere,
so we define our own version that's equal to what UNIX
typically has.

The bulk of this patch was contributed by Mohit Aron.

R=jeff


git-svn-id: http://snappy.googlecode.com/svn/trunk@76 03e5f5b5-db94-4691-08a0-1a8bf15f6143