implement ActiveRecord::Base#pretty_print

AR::B has long been lacking pretty print support. I submitted this patch some years ago (pre-github), attempting again. 

before:

```
pp topic

#<Topic id: 1, title: "The First Topic", author_name: "David", author_email_address: "david@loudthinking.com", written_on: "2003-07-16 14:28:11", bonus_time: "2000-01-01 14:28:00", last_read: "2004-04-15", content: "Have a nice day", important: nil, approved: false, replies_count: 1, unique_replies_count: 0, parent_id: nil, parent_title: nil, type: nil, group: nil, created_at: "2014-05-18 21:33:26", updated_at: "2014-05-18 21:33:26">
```

(same as #inspect)

now: 

```
pp topic

#<Topic:0x007fb93696bcb0
 id: 1,
 title: "The First Topic",
 author_name: "David",
 author_email_address: "david@loudthinking.com",
 written_on: 2003-07-16 14:28:11 UTC,
 bonus_time: 2000-01-01 14:28:00 UTC,
 last_read: Thu, 15 Apr 2004,
 content: "Have a nice day",
 important: nil,
 approved: false,
 replies_count: 1,
 unique_replies_count: 0,
 parent_id: nil,
 parent_title: nil,
 type: nil,
 group: nil,
 created_at: 2014-05-18 21:31:53 UTC,
 updated_at: 2014-05-18 21:31:53 UTC>
```