Add support to use certificates from string instead of file

[zalmane]

Currently, redis-py support connecting using SSL certificates stored in files by passing the certificate path to the redis connection. This, in turn, is sent to the python/cython SSL module.
Python 3.4 added support for loading certificates from a string in addition to a file. See https://docs.python.org/3/library/ssl.html#ssl.SSLContext.load_verify_locations.
Supporting the parameter cdata in redis-py connection will enable to send certificates from a string instead of a file.

Version: What redis-py and what redis version is the issue happening on?
All

Platform: What platform / version? (For example Python 3.5.1 on Windows 7 / Ubuntu 15.10 / Azure)
All

Description: Description of your issue, stack traces from errors and code that reproduces the issue