Allow setting an error handler

This changeset introduces phpiredis_set_error_handler(), which allows a user to register a PHP callable as an error handler. This handler is called when an error is encountered in the communication with the Redis server. It should correspond to the following signature: function ($errorType, $errorMessage)

$errorType is one of PHPIREDIS_ERROR_CONNECTION and PHPIREDIS_ERROR_PROTOCOL, $errorMessage is just the message as a string.

I am already opening this pull request to get some input as to whether this approach is fine for everyone. Also my C isn't the best, so please let me know if there is a mistake somewhere.

Still left to do:
- Call the error handler in phpiredis_multi_command_*() as well.
- Call the error handler for connection errors as well.
- Maybe pass the returned value from the error handler back?