ContextLogger: evaluate show_stack later

Sometimes a ContextLogger is already instantiated before the log level
is set to DEBUG. This would cause stack information not to be displayed.
Change it so that we evaluate isEnabledFor(DEBUG) just before emitting a
log record.
Revert the removal of switch_back.throw()
flake8 fix
Improved manual!
Add tests for the examples.
Run flake8 tests on all sources including tests.
Remove txdbus tests.
Remove .egg-info directory before making a release.

I've noticed that the 'SOURCES.txt' file in this diretory sometimes
contains a stale file list. So to be sure, we remove the entire
directory and have setuptools recreate it.
Bugfix in Pool.submit()

Use Queue.put_nowait() because submit() should never wait.
Also show exception value in pyuv tracebacks.
Comment out debug() in selected places when running with python -O

Gives a 3% performance boost in some benchmarks.
http: fix HTTP/1.0 keepalive
http: optimize some more

This change makes HttpProtocol use a static http_parser_settings rather than
creating a new one for every connection.
Add "tox" and "detox" dev requirement.
http: optimizations