Introduce generic property collection in issue JSON document

#### DESCRIPTION

It's suggested to change issue JSON document as follows:

```json
{
   "properties" : {
      "property1" : "value",
      "property2" : "value"
   }
}

```

These values need to be persisted by `CodeReview.Evaluator` and other data import tools. 

Primary use of this feature is to unified storage of custom data for different converters.
