Add Module Maps to .framework

In order for the generated OpenWebRTC.framework to work with CocoaPods, and that users should be able to add it as `@import OpenWebRTC;`, Module Maps should be added. Hopefully this removes the need to add a custom HEADER_SEARCH_PATHS after the framework has been installed.

Some more info is available here:

http://blog.cocoapods.org/Pod-Authors-Guide-to-CocoaPods-Frameworks/

Start by tracing how the generated OpenWebRTC.framework is assembled and where its headers and metadata are placed. Read the linked CocoaPods module-map guidance and verify the framework can be consumed through CocoaPods with @import OpenWebRTC; done means no custom HEADER_SEARCH_PATHS are needed.

Written by the indexing model from the issue text.