Post model has uid property

The pagination of the posts is based on the id of the last post received, so we need to track it as well.
Implemented parsing of the post id
Added method to request posts given a pagination reference

To do this I refactored STRAppDotNetProxy by extracting a more configurable method from the only get posts method present before. This "base method" is the only one that is tested now, because the other two (getPosts, and getPostsAfterPost) just call that under the hood.