[SDK] [iOS] Release v1.2.1.0

GitHub release
Integration guide

Changelog:

• accessToken getter
• getInstallationCustomProperties
• Fixed user switching
• Properly initialize setNotificationEnabled: from pre-WonderPush application versions
• Better setNotificationEnabled: documentation
• Refresh core properties and device token if access token changes
• Attach notification info to trackEvent actions
• Fix trackEvent method signature
• More type checks - fixes #2
• Remove a few HTTP requests
• Upgrade to AFNetworking 3.1.0 and DTFoundation 1.7.10