Add Persistable query builder methods

This adds methods for querying either `Persistable` objects:

* `Persistable.findByQuery(queryBuilder, handler)`: Queries for a single object.
* `Persistable.findAllByQuery(queryBuilder, handler)`: Queries for a many objects.

Example usage:

```swift
User.findByQuery({ (query: PFQuery) -> Void in
    query.whereKey("userId", equalTo: facebookUser.objectID)
}, handler: { (user, error) -> Void in
    // Handle the response
})
```

@snekkalapudi this also adds a `User.findOrCreateUserForFacebookUser` method. I was getting an error in Parse when trying to fetch a user's stories, because the initialized user didn't have a Parse objectId. This new method will return a `User` object that has been persisted to Parse.