Add support to search for Observations by Observation.code

We need to support the ability to search for observations using observation.code

observation.code is of type 'token' (see 2.2.2.5 token of http://hl7-fhir.github.io/search.html#token)
We need to be able to support
GET/Observation?code=[namespace]|[code]
Example: GET/Observation?code=http://loinc.org|8302-2
as well as
Example: GET/Observation?code=http://loinc.org|8302-1,http://loinc.org|8302-2,http://loinc.org|8302-4

The scope of this ticket is limited to OR operations. But we will later expand to AND operations as well.
In order to support future work using AND, we must build the initial work using HAPI's TokenAndListParam

Please refer to http://jamesagnew.github.io/hapi-fhir/doc_rest_operations.html#Multi-Valued_ANDOR_Parameters for further information