PostgreSQL Audit Extension

The attached patch implements audit logging for PostgreSQL as an
extension.  I believe I have addressed the concerns that were raised at
the end of the 9.5 development cycle.

There are some caveats:

1) This patch depends on the ereport-v1 patch that has also been
submitted to the 2016-01 CF.  The ereport patch allows audit log
messages to be suppressed on the client which has been a common ask.  If
the ereport patch is not accepted then the same functionality could be
implemented with elog callbacks or removed.

2) The included documentation is currently in Markdown format.  It was
converted for the community project but I'll convert it back to SGML.

3) 'make installcheck-world' isn't going to work on the build farm
because of the shared_preload_libraries requirement.  'install-check' on
the extension works fine if pgaudit is added to shared_preload_libraries
before running - I'll be happy to help look at this issue.

I would also like to thank Abhijit Menon-sen and Ian Barwick of
2ndQuadrant who authored the version of pgaudit that this work has been
based on.

--
-David
david@pgmasters.net