Updated MGET response handler, added a helper function
Initial commit of MSET and MSETNX commands.

More work needs to be done to work through what, exactly, to do
in the case of a protocol error in commands like this.  If Redis
(or communication with Redis) bombs out halfway through a MSETNX
reply handler, we will have alredy sent the commands but not yet
consumed all of the responses.

This would put RedisCluster in an inconsistent state, and we have
a few options.

1.  Abort immediately
2.  Continue to process responses, but php_error_docref that it
    occured and use FALSE to indicate a failure occured.
3.  Stop processing responses, and reset the connection to Cluster

More testing will need to be done to figure out what the best
stragegy is.
DEL command implemented

Modified our MSET handler and made it generic (it has the same
general mechanism as DEL, just with a different return type)
and implemented the DEL command.

We've got the same consideration to make.  What to do in the
event of a catastrophic failure.
Return getThis() pointer from MULTI
Refactoring, updated handling of MULTI commands

Minor formatting changes

Updated how we delete context variables, such that our context
pointers aren't always going to be a zval** array.  This
introduces the possibility of creating a memory leak if we have
to abort a transaction due to a communications error, but this
can be handled later by attaching a void dtor(void*) callback.