Populate

Hi Anthony, 

here is my Populate test. 
I can not assure that this does'n break anything else, because i can't get your $pullAll-Test to work. 

```
  1) Mockgoose Update Tests $pullAll should be able to pull multiple items from nested documents array by property
   Message:
     timeout: timed out after 5000 msec waiting for spec to complete
   Stacktrace:
     undefined
```

I check the object in the contains function  and transform it into a json string. The test will populate the users inside the company model. 
I will add more tests when i get rid of the error above. 
If i checkout your master, than npm test throw a error:

```
TypeError: Cannot read property '_id' of undefined
    at Promise.<anonymous> (~/Downloads/Mockgoose-master/test/Find.spec.js:22:34)
```