Added a data model to the project

Right now I've just got the user entity in there.
Replaced current HKUser implementation with CD managed version
HKUser now fully unpacks the API's response dictionary
Reorganised files a bit
Fixed some bugginess in user persistence

It's kind of hacky for now, this could use a refactor
Started documenting HKManagedObject
Added some notes on subclassing HKManagedObject
Documented HKRemoteManagedObject
Added notes on subclassing HKRemoteManagedObject
Added documentation for HKUser
Fixed a few mistakes in the documentation
Fixed some missing docs for HubKit.h
Some initial docs for the HubKit class
Added docs for HKUser
Completed missing docs for HKHTTPClient
All documented :boom:
Merge pull request #36 from rpwll/core-data

Reimplementing Core Data
Merge pull request #37 from rpwll/docs

100% Documentation Coverage