failing tests for empty unsub and punsub

When unsubscribe or punsubscribe is called
and there is nothing to unsubscribe from, the reply[1]
argument is a null which causes a TypeError
Cannot call method 'toString' of null

```
TypeError: Cannot call method 'toString' of null
    at RedisClient.return_reply (/Users/barczewskij/projects/node_redis/index.js:633:65)
    at ReplyParser.RedisClient.init_parser (/Users/barczewskij/projects/node_redis/index.js:266:14)
    at ReplyParser.EventEmitter.emit (events.js:96:17)
    at ReplyParser.send_reply (/Users/barczewskij/projects/node_redis/lib/parser/javascript.js:300:10)
    at ReplyParser.execute (/Users/barczewskij/projects/node_redis/lib/parser/javascript.js:211:22)
    at RedisClient.on_data (/Users/barczewskij/projects/node_redis/index.js:483:27)
    at Socket.<anonymous> (/Users/barczewskij/projects/node_redis/index.js:82:14)
    at Socket.EventEmitter.emit (events.js:96:17)
    at TCP.onread (net.js:396:14)
```
fix empty unsub/punsub TypeError

When unsubscribe or punsubscribe is called and it has
no subscriptions, the reply[1] is a null which causes
`TypeError: Cannot call method 'toString' of null`

Check if reply[1] is null before calling toString otherwise
just pass null.
fix bug when callback is undefined
limit cbtests to 2.6.11 and above

Test hangs on older versions of Redis
fix unescaped exception in Multi.exec
handling of errors on multi, now returns 'err' array type
added test for MULTI exception
update docs for MULTI exception
Adding missing 0.8.3 changelog
Deprecate end() by having it call quit() instead. Marked for eventual removal. Fixes #419
Merge branch 'thanpolas-fix-exception-in-multi-exec'
Merge branch 'jeffbski-0.8.2-unsub-empty'
Merge branch 'jifeng-master'
Merge 'mranney/master' (up to 747cd1f10)

Conflicts:
	README.md
	index.js