Adding a clearer error message when trying to use an invalid includes.

Before the error was this:

```
TypeError: Cannot read property 'model' of undefined at
utils.mixin._createSelectStatement
```

Now it is this:

```
Error: Could not find the associated model "BadAssociation" on the
"GoodModel" model. at utils.mixin._createSelectStatement
```

Test added as well.
Merge pull request #172 from JohnPostlethwait/invalid-includes

Adding a clearer error message when trying to use an invalid includes.