* BaseClient and AsyncClient
* Introduce 'httpx.Client'
* Top level API -> sync
* Top level API -> sync
* Add WSGI support, drop deprecated imports
* Wire up timeouts to urllib3
* Wire up pool_limits
* Add urllib3 proxy support
* Pull #734 into sync Client
* Update AsyncClient docstring
* Simpler WSGI implementation
* Set body=None when no content
* Wrap urllib3 connection/read exceptions