Make use of format to infer type of generated fields

[HaukeRa]

The generator only uses the type field to determine the type of the generated field in a class.

For example this property:

    "createdAt": {
      "type": "string",
      "description": "The timestamp in milliseconds when the account was created.",
      "format": "int64"
    }

will generate the field val createdAt:String despite the format giving the hint, that it should actually be interpreted as an integer. The same issue arises when a field is declared with "type":"number" while having "format":"int64". The generated classes use BigDecimal in this case, while a Long would be way more convenient and can be ensured to work due to the format.

Note: int64 can be replaced with int32 without changing the issue.