Added DISCARD

I originally didn't think DISCARD would do anything here because of the clever MULTI interface, but somebody
pointed out to me that DISCARD can be used to flush the WATCH set.

Added missing HVALS command

Fix bug with empty Array arguments - Andy Ray

Add `MONITOR` command and special monitor command reply parsing.

Fix bug in ready check with `return_buffers` set to `true`.

Thanks to Dean Mao and Austin Chau.

Add probe for server readiness.

When a Redis server starts up, it might take a while to load the dataset into memory.
During this time, the server will accept connections, but will return errors for all non-INFO
commands.  Now node_redis will send an INFO command whenever it connects to a server.
If the info command indicates that the server is not ready, the client will keep trying until
the server is ready.  Once it is ready, the client will emit a "ready" event as well as the
"connect" event.  The client will queue up all commands sent before the server is ready, just
like it did before.  When the server is ready, all offline/non-ready commands will be replayed.
This should be backward compatible with previous versions.

To disable this ready check behavior, set `options.no_ready_check` when creating the client.

As a side effect of this change, the key/val params from the info command are available as
`client.server_options`.  Further, the version string is decomposed into individual elements
in `client.server_options.versions`.

Fix excess memory consumption from Queue backing store.

Thanks to Gustaf Sjöberg.

Fix multi/exec error reply callback logic.

Thanks to Stella Laurenzo.

Fix bug where unhandled error replies confuse the parser.

Fix bug where subscribe commands would not handle redis-server startu…

…p error properly.