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
Merge pull request #36 from rpwll/core-data

Reimplementing Core Data