add callbacks for mongoose remove method

Article tests were failing.

```
<Unit Test> Model Article: "before each" hook:
Error: timeout of 2000ms exceeded
at null.<anonymous> (/Users/andra/trash/mean/node_modules/grunt-mocha-test/node_modules/mocha/lib/runnable.js:139:19)
at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)
```

After looking closer at the `beforeEach` and  `afterEach` calls, we stumbled across this gem in the [mongoose docs](http://mongoosejs.com/docs/api.html#query_Query-remove):
_"The operation is only executed when a callback is passed. To force execution without a callback (which would be an unsafe write), we must first call remove() and then execute it by using the exec() method."_

It seems like those removes were never actually being executed properly! Supplying callbacks seems to have fixed the test.

Thanks to @andrija-hers for his help finding and diagnosing the bug!