Searching with operators other than "equals"

I added Cql::Model#where to accept a hash or string with optional values.  The following will all work:

```ruby
Person.where(first_name: 'John', last_name: 'Doe')
Person.where("first_name = 'John' AND last_name = 'Doe'")
Person.where("first_name = ? AND last_name = ?", ["John", "Doe"])
```

Among other things, this allows the ability to use operators other than "equals", which is currently all that #find_by supports.  In the future, we can also add optional parameters to account for CQL options like ALLOW FILTERING.