Add getArgument($index) in Predis\Command\CommandInterface.

This method should have been part of the interface since start since
it is used through the library. We also do not specify a default value
for the index argument since it does not make much sense.
Use hexadecimal notation.
Remove a few more unnecessary strict comparisons.
Use a nicer looking version for the same code.
Prevent E_NOTICE messages on __destruct() if $parameters is not set.

This can only happen when throwing on invalid connection parameters,
this is a quick fix that will do for now.
Deprecate the whole Predis\Helpers class.