Added PCL lib

I pulled out Interfaces for all public classes in the Shared code to create Couhchbase.Lite.Portable  (PCL). This PCL can be used to create PCL business logic libs, and used with Xamarin Forms.

I haven't created an example using Xamarin Forms yet, but the existing examples could be ported to a forms example.  I have not tested sync with server, but local functionality works in both iOS and android tests, but the only changes are to support use of the portable interface definitions.

I created this to suport a project I am working currently and and I need to support multiple platforms with portable business logic.  I also commented on the couchbase forums. http://forums.couchbase.com/t/trying-to-build-couchbase-lite-ios/2019