Add timeout and log-capturing py.test plugins
Lots of work on querystring parser.

- Max length restrictions now do what they're supposed to
- Support for None values in Field() classes
- More logging
- Strict parsing is now an option for QuerystringParser
- Remove "keep blank values" flag, since we can't actually deal with it
  without having some sort of buffering in the parser, which I won't be
  adding.
Fix some bugs in threadlocal properties

They now handle other threads (where the threadlocals haven't been
initialized) better.
More tests to reach 100% test coverage
Add test that checks threadlocals across threads, fix bug
Misc. fixes for Py3, update tox.ini