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Make Predis v0.8 compatible with HHVM 2.4.0.

Since we cannot rely on the PHP version to detect the availability of
socket_import_stream(), we switched to function_exists(). As an added
bonus, using function_exists() is twice faster.
Update README about HHVM compatibility.
Update CHANGELOG.
Add support for key hash tags in redis-cluster (Redis 3.0.0b1).

Multi-keys operations are not allowed even when keys generate the same
hash but this will probably be supported in later betas of Redis which
means we will basically end up reusing the whole strategy used for
client-side sharding.