Added CategoryService to house all methods from original NSMO
Refactored to use CategoryService rather than class methods on Category
Refactored CategoryService to place remote calls in a separate class
Make sure to set server's category ID on local Category entity
Be nicer if the NSManagedObjectID is nil - just return nil
Make parent Category 0 if not set and convert incoming ID to a NSNumber
Get rid of the background context in CategoryService

If a background context is required for merging, than a new instance of CategoryService should be created in the calling code using a background context.
Fixed a misspelled class name in the Core Data model for Category
Cleaning up extraneous comments left from previous architecture
Merge branch 'develop' into issue/1410-service-layer

Conflicts:
	WordPress/Classes/WPAddCategoryViewController.m
Merge pull request #1411 from wordpress-mobile/issue/1410-service-layer

Refactor Category to remove network & business logic methods