Add Predis\Connection\PhpiredisStreamConnection.

This class works just like Predis\Connection\PhpiredisConnection but
it does not require the socket extensions since it relies on PHP's
native streams thus allowing the use of persistent connections.
Update link to phpiredis.
No need for "iterable_multibulk" as a default connection parameter.

Furthermore, connections that do not support certain parameters should
check and throw whether they are set, independently from their value.
Tweak README.
Update FAQ.