Carthage, older platforms, max articles count

• Configurable maxArticleCount in Get articles operation
• Carthage support
• Support for older platforms
• Login operation does not return error in case of missingData error
• Add pod lint as a step of the build script
• Cover PPTransport with tests
• Replace HTTP methods strings with enums
• Minor code style corrections in operations
• Enforce completion closures for operations (Public API change)
• flatMap instead of forEach
• Minor stubs naming and style corrections
• Delete OWNERS file
• Update example's podfile to the latest version