Adding a script to verify the tests against node_redis and creating a unified Item

I've done three main things:
- Added a way to run the tests against node_redis to check that our tests are correct. To do so run `make check-tests` I've also added the script directly into the travis config.

- Fixing #13 and my previous pull request. I've created a unique item and a factory to create string, hashes and lists. 

- Creating a method `_callCallback` in RedisMock to DRY the lines
```js
if (callback) {
  process.nextTick(function () {
    callback(err, result);
  });
}
```