new release: 1.5b15

Mathopd 1.5b15 has been released.

Changes with respect to the last beta:

- some parts of the documentation have been updated;
- tiny efficiency hack in the spawn function;
- rewritten the routines that output things to the client;
- fix some conformance problems with RFC2616.

b15 fails to build with the Sun C compiler. Unfortunately I already built
the release, but this will be fixed in the next release. Meanwhile use gcc
instead (or fix manually - should be trivial)