Added whereEquals(array) and findBy(array) to query builder and Eloquent. 

This PR replicates a feature I liked in Doctrine, where records or models could be retrieved by providing an array of where equals columns and values. For example:

```php
$user = User::findBy(['name' => 'Bob', 'job' = 'Builder']);
```

and to add the constraints without returning a single result:

```php
$users = User::whereEquals(['age' => '37', 'job' = 'Builder'])->get();
```

Tests are provided.

Thanks!