Calling kue.Job.get() with undefined or null causes uncaught exception

Calling `kue.Job.get()` and passing an undefined or null `id` causes an uncaught exception:

```
FATAL: UncaughtException: Cannot read property 'toString' of undefined TypeError: Cannot read property 'toString' of undefined
at Redis.client.createFIFO (/app/node_modules/kue/lib/redis.js:82:26)
at Function.exports.get (/app/node_modules/kue/lib/queue/job.js:173:20)
```

from this entry point: https://github.com/Automattic/kue/blob/master/lib/queue/job.js#L161

the `get()` function does `job.zid = client.createFIFO(id);`.

The uncaught exception happens on this line in `createFIFO()`: (https://github.com/Automattic/kue/blame/master/lib/redis.js#L82)
```
var idLen = '' + id.toString().length;
```

Granted, it shouldn't be passing in `undefined` as `id` to `Job.get()` initially, and I can add a check to make sure that doesn't happen in my app code.

That being said, it might be better not to allow this use case to generate an uncaught exception in the Kue code, and instead call the return fn with a validation error

Read lib/queue/job.js around Job.get() and lib/redis.js around createFIFO(), then reproduce the null and undefined calls. Done means those inputs no longer cause an uncaught exception and the return callback receives a validation error.

Written by the indexing model from the issue text.