Dependency support + CocoaPods 

Hi Saul,

Here are some updates to the project. 

Summary:

- Child commands are now send to the bus in order of addition to the parent command (NSSet -> NSArray)
- Added support for dependencies between operations (useful if you want to execute child operations in a certain order)
- Fixed batch operations so that the completion methods for sub commands are now called.
- Conditional compilation for iOS/OS X parts (NS/UIApplication categories).