feat(case/REST): create a new case instance via REST Api

This commit adds the possibility to create a new case instance via REST
API:

POST /case-definition/aCaseDefId/create

Request Body:

  {
    "businessKey" : "aBusinessKey",
    "variables"   : {
    		      "aVariable" : {
		                       "value" : "aStringVariable",
				       "type"  : "String"
				    },
	              ...
		    }
  }

Furhtermore this commit exposes the CaseInstanceQuery and
CaseExecutionQuery via REST API.

related to CAM-2188