ImageDownloader ACL Update and tvOS Fix

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Added

• Note to the README about storing a strong ref to image downloaders.
  • Added by Muhammad Ishaq in Pull Request
    #45.
• Custom Info.plist for tvOS setting the UIRequiredDeviceCapabilities to arm64.
  • Added by Christian Noon.

Updated

• The sessionManager ACL in the ImageDownloader to allow access to the underlying
  session and configuration.
  • Updated by Christian Noon.
• The Alamofire submodule to the Alamofire 3.1.3 release.
  • Updated by Christian Noon.