Implemented adding categories in REST

CategoryService was refactored to make use of RemoteCategory objects.

The add category view controller no longer syncs categories after
creation since REST will already return the new category. When sync is
moved to CategoryService, we need to trigger it after creation for
XML-RPC remotes.
Refactor category creation

If the user tries to create a category with a name that already exists,
let's pretend we just created it and use the existing category instead
of failing.
Moved category syncing to CategoryService

Implemented REST category sync
Force blog sync on blog details view
Sync categories after creation for XML-RPC
Merge branch 'develop' into issue/2551-category-service
CHange assert to check for empty strings besides nil
One class per line
Removed unnecessary check
Merge branch 'develop' into issue/2551-category-service
Merge pull request #2570 from wordpress-mobile/issue/2551-category-service

#2551 REST Cleanup: Categories