Adds ReaderTopics model and services.

Basic implementation that does not yet establish relationships with other models (e.g. ReaderPosts).
ReaderTopics: Better dictionary normalization. Use NSNumber for ID.
Let ReaderTopicsViewController use ReaderTopicService
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/1327-reader-endpoints
Merge triage.
First pass at ReaderPostService architecture.

Establishes relationships with ReaderTopic.
Does not yet remove old ReaderPost logic from the model.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/1327-reader-endpoints
Removes code related to the old reader endpoints.

Updates likes, follows, reblogs, etc to use the service.
Establishes proper relationships between topics and posts.
Removes obsolete properties from ReaderPost model.
Removes obsolete code from ReaderPost model.
Updates notifications and the app delegate to call the new service.
Corrects a couple of issues marshalling values from the REST API results.
Handle html entities in site titles.
Fixes an issue with loading more posts in the reader.

Tweaks type of some contants.
Fix an issue where the no results view did not clear on first load after install.

Use derived contexts instead of the main context with the post service.
Move the save calls in the services into the performandwait blocks for clarity.
Create relationship between topics and accounts.

Cleans up topics when changing accounts
Make sure merges are performed in the background to not block the main thread.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/1327-reader-endpoints
BOOLs not NSNumbers
Use NSObjects instead of NSDictionaries to represent remote REST objects.

Gets ride of the NSDictionaries used to represent ReaderTopic and ReaderPost objects to the local services.
Instead uses plain objective c objects with properties.

Also cleans up a one sided relationship between ReaderPosts and Accounts that was missed in an earlier commit.
Tests for reader services.

This is a first pass at tests for the reader service.
It is not exhaustive.  Some things that could have been tested have been omitted
due to planned changes in the reader code, other things will be added over time.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/1327-reader-endpoints
WPTableImageSource now resizes its images maintaining transparencies - #1100.
Decode xml entities in topic titles.