[#656] Use a Text property for values longer than 500 chars

* Introduce a new property `valueText` that is used to store the
  response value for large strings (more than 500 chars)
* The `setValue` and `getValue` methods check for the properties `value`
  and `valueText` for storing and reading the value
Merge pull request #657 from akvo/issue/656

[#656] Use a Text property for values longer than 500 chars