Convenience Methods

This PR adds a little dash of convenience.

### Return the First Result

A wrapper for setting `limit` to 1 and calling `#get`.

```ruby
first_user = Parse::Query.new('_User').first
```

### Batch Update Attributes

Adds a new method, `#update_attributes` to `Parse::Object`.

```ruby
user = Parse::Query.new('_User').eq('email','joe@example.com').first

# destructively update multiple attributes at once
user.update_attributes email: 'john@example.com', name: 'John Smith' 
```

### Use a Hash for `eq` Queries

Allows the `#eq` method to accept a hash of key/value pairs to construct an AND query.

```ruby
user = Parse::Query.new('_User').eq(email: 'john@example.com', name: 'John Smith').first
```