Issue Metadata support

[akx]

Do we need to support arbitrary extra fields for requests, i.e. the full spec here?

http://wiki.open311.org/GeoReport_v2/#get-service-definition

(We could push that to a later version.)

[aapris]

Let's do this later, currently it is not supported by CitySDK anyway.
http://dev.hel.fi/apis/open311/#GET%20Service%20Definition