Find with condition

Adds predicate support to find:

``` objective-c
[Person find:@"firstName = 'Super' AND lastName = 'Marin'"];
```

And a helper:

``` objective-c
[Person findWithFormat:@"firstName = %@ AND lastName = %@", @"Super", @"Marin"];
```

The third commit switches things to use `predicateWithFormat:`, which COULD be a breaking change for some users, but should provide a more stable API going forward.