Fix value converter - Githubissues

I've updated this PR with further improvements to the Value Converter

Updating value converter to return single or collection based on multipleItems prevalue
Updating dependency to Umbraco v7.1.5 so that IPropertyValueConverterMeta can be implemented and implementing it
Switching to ConvertSourceToObject as that is what this converter is returning a object not a source

kgiszewski / uWestFest