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
Started extracting some media sizing code out of WPMediaProcessor.

Created WPMediaSizing class to deal with sizing issues that
WPMediaProcessor shouldn't be dealing with. Pulled
mediaResizePreference method out.
Moved sizeForImage out of WPMediaProcessor and into WPMediaSizing.
Moved resizeImage out of WPMediaProcessor and into WPMediaSizing.
Cleaned up loop that processes images from image picker.
Moved WPMediaProcessor metadataForAsset method to WPMetadataExtractor.
Moved image save methods out of WPMediaProcessor into WPMediaPersister
Removed WPMediaProcessor and extracted remaining functionality out.

Removed WPMediaProcessor as it's no longer necessary since we split up
the functionality of the class into a few methods. This commit removed
the final functionality which dealt with the uploading of the media.
Created WPMediaUploader to contain this functionality.
Added initial media progress upload hooks.
Added a media progress indicator to the navigation bar of the editor.
Fixing bug where selecting multiple images uploads duplicates.

The problem was that we were tying the filename to the date forgetting
that selecting multiple images will result in the same filename.
Adjusted the file naming mechanism to take nanoseconds into account.
Adding an activity indicator to the media uploading view.
Removing unused method in WPMediaUploader
Moving correctlySizedImage method to WPMediaSizing.