Add StringDictProperty

Ensures that all dict values are string types. The DictProperty
converts the value '802.11' (see example below) into a Decimal
property. The StringDictProperty prevents this conversion.

Example:

  {
    "name" : {
      "de" : "Stadt",
      "en" : "City",
      "xx" : "802.11"
    }
  }
Merge pull request #163 from dfa/string-dict-property

Add StringDictProperty