Merge predis.default_parameters even when parameters are URI strings.

Too bad Predis\Connection\ConnectionParameters::parseURI() is private
forcing us to copy and paste its implementation here.
Remove useless stuff.
Improve multiple clients example to use default parameters and options.
Add example of how to use Redis for handling PHP sessions.

Predis v0.8.2 is required to use Predis\Session\SessionHandler.