Breaking changes in v2.x

Breaking changes in v2.x

• Removed attributes from writeFile and appendFile for iOS / Android consistency
• downloadFile takes options object rather than parameters
• stopDownload will cause the rejection of promise returned by downloadFile
• uploadFiles promise result response property is now body
• A boolean is no longer returned from any method except exists
• downloadFile and uploadFiles return an object of the form { jobId: number, promise: Promise }