Fix #197, adds NSCoding support for MCOAddress, load body preview in iOS example app

1. Fixes issue #197: Build error: 'MailCore/MCIMAPMessageRenderingOperation.h' file not found
2. Adds NSCoding support for MCOAddress
3. Updated iOS example app to:
   * Load the last 10 messages
   * Load and display a preview of the loaded messages in the subtitle