Implement UnixTimestampJsonConverter, a DateTime converter for Eastern Time Zone

Added decoration for this converter to Severity.Created, Severity.LastModifier
EffectPeriod.Starts and EffectPeriod.Ends

Thanks to Andrew Whiteaker who solved my issue of UnixTimestampJsonConverter
http://stackoverflow.com/questions/26667664/newtonsoft-json-custom-jsonconverter-deserialize-to-datetime-not-working