libpqxx: great renaming

To prevent future confusion I've just unified the naming structure for
libpqxx (an alternative C++ API for PostgreSQL). This means that any
code using the library will need some editing before it will compile
again.

The following changes were made:

- The "Pg" namespace was renamed to "pqxx" to match the library's name.
This arguably isn't as pretty, and it adds 2 characters, but it's a
damn sight more consistent.

- Header files were renamed from "pg_*.h" to "pqxx/*.h" to match the
above change. It also replaces an identifying prefix by a syntactic
construct, which is exactly what the namespace does for the code.

- Separate directories were created for library header files, library
source files, test program sources, and the library in binary form.
I wouldn't call this work complete (object files don't even go into
their own directory yet) but hopefully it will pave the way for the
future. It also reduces clutter.

These changes will require changes in both installation and code, which
I thought would be more acceptable for an experimental version than for
production code. I don't expect any other changes to be needed to get
existing code to run with the new version. Also, the changed namespace
should help trip up any code still accidentally relying on the old code.

In the future I hope to take a more detailed look at method names and
see if any changes are needed to harmonize their naming styles. An
example that springs to mind is name() vs. Name().

Get the latest version from the libpqxx home page:

http://members.ams.chello.nl/j.vermeulen31/proj-libpqxx.html

A notice about the incompatible changes has been put up to warn current
users.

Jeroen