Change documentation to include the full package path.

Classes which used to be called txpostgres.Connection are now called
txpostgres.txpostgres.Connection and so on. Even it more verbose, it's also
closer to the truth and does not require awkward adding of a non-package to the
system path when building the docs.
Clean up some old comments and fix whitespace formatting.
Implement automatic reconnecting.

Two new modules got added, one for retrying calls with exponential backoff and
another for detecting database disconnects and reconnecting.
Add tests for the new modules implementing automatic reconnecting.
Document the automatic reconnection feature and add example code.
Add files generated by coverage to gitignore.
Mention automatic reconnection in the NEWS file.
Add seemingly required license mumbo-jumbo.

The retrying.py file seems to be covered by the Apache License 2.0 which
requires including its full text in the distribution (so be it).