Progress on search
Some search documentation
Implement `:exact` and `:text` search specifiers
README update
Fix `exact` and `text` modifiers
Move `contained` and `language` to the base resource class
Fix DocumentReference
Add a referenced resource resolver

Only implemented for "singular" references (medication = Medication), not yet for arrays (author = [Practitioner, Device, Patient, ...]).
Add `public` declaration introduced with beta 4
Implement ISO date parsing. Fix numeric value JSON decoding.

Numbers in the JSON don't come back as `String` but as `Double`, use this fact to change the NSDecimalNumber convenience initializer. Needs more testing but works with quantities found in observations.
Update for beta 5