Performance improvement for numerics

Added a performance test for numerics and before the speed-up it took
about 2.6 seconds. It's now taking about 0.4 seconds.

Moved the 'tests' directory into the 'pg8000' package to make the
performance tests easier to run.

PEP8 and pyflakes tidy-up on types.py.

Added README.creole for displaying on GitHub.