Use maven-ant-tasks to fetch waffle-jna

We need Waffle and its underlying JNA dependencies in order to support
SSPI authentication on Windows. We compile that support on all platforms,
it just only actually works when running on a Windows host.
Add SSPI support to PgJDBC

This change will cause code that was previously attempting direct SSPI
negotiation against a Kerberos server to instead use Windows' SSPI support
if it is available.

SSPI support using waffle-jni is always compiled, but the libraries are
not included in the default PgJDBC jar. They must be added on the classpath
or included in a "fat jar" for SSPI to work. When missing, PgJDBC throws
an informative exception.

TODO: In case anyone is relying on doing Kerberos negotiation with a PostgreSQL
server in sspi authentication mode I'll need to add a URL parameter to force
the old mode to be used, and to default to that if Waffle isn't available or we
aren't on Windows. Or only use the new mode when explicitly requested by a URL
parameter.

TODO: Fall back to GSSAPI on !windows
Build "fat jar" optionally
Copy maven-ant-tasks dependencies to lib/ for use in Eclipse etc
Backwards compatible negotiation of SSPI use
Prototype of better SSPI negotiation