specify infoMimeFormats pr layer and add support for JSON feature info

In a wmsLayer cache, I want to have support for GetFeatureInfo with application/json only. This pull request adds support for an optional infoMimeFormats-element for each wmsLayer. This information is checked upon during request and included in the capabilities output.

    <wmsLayer>
     ....
      <infoMimeFormats>
        <string>application/json</string>
      </infoMimeFormats>
    </wmsLayer>