Implementations of chrome.system.* for ios (cpu, display, memory, network)

- Add auto tests for chrome.system.cpu plugin
- Initial stub implementation of chrome.system.cpu.getInfo for ios
- Add auto tests for chrome.system.display plugin
- Implementation of chrome.system.display.getInfo for ios
From what I've been told, you should avoid exceptions in Obj-C (forget the rationale right now though).