-------------------
- Add Client.set_default_soapheaders() to set soapheaders which are to be used
on all operations done via the client object.
- Add basic support for asyncio using aiohttp. Many thanks to chrisimcevoy
for the initial implementation! Please see
https://github.com/mvantellingen/python-zeep/pull/207 and
https://github.com/mvantellingen/python-zeep/pull/251 for more information
- Fix recursion error when generating the call signature (jaceksnet, 264)