CRM-13023 - API Case - Allow Get method to be called with no parameters ...

...or with entity fields as normal

Includes the following changes:

* Removed the verification for the mandatory parameters
* If none of the previous parameters are found it calls the standard _civicrm_api3_basic_get method to get the Case based on the field values provided in $params.
* Then use the _civicrm_api3_case_read method to get the case details so they match the format returned by the other branches of the get method.

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* CRM-13023: API Case Entity and Get Action can not be called without search fields
  http://issues.civicrm.org/jira/browse/CRM-13023