Add removesKeysWithNullValues setting to serialization and copying.

Preserve removesKeysWithNullValues through copying and serialization.
CocoaDocs colour scheme

Think this goes pretty well your AFNetworking.com branding. You're welcome to go make your own at http://cocoadocs.org/readme/

![http://cl.ly/image/2Z2c0h390L0M/Screen%20Shot%202014-07-01%20at%2010.20.36%20PM.png](http://cl.ly/image/2Z2c0h390L0M/Screen%20Shot%202014-07-01%20at%2010.20.36%20PM.png)
Merge pull request #2168 from orta/patch-2

CocoaDocs colour scheme
Merge pull request #2162 from jshier/master

Add removesKeysWithNullValues to serialization and copying.
Ensure copyWithZone: calls the parent imp

AFURLConnectionOperation implements NSCopying, so we need to call the
parent's version of copyWithZone: to ensure the object is properly
copied.
Merge pull request #2172 from streeter/copywithzone-in-subclasses

Ensure AFHTTPRequestOperation's copyWithZone: calls the parent implementation