Refactored out HttpClient sending from JsonNetStronglyTypedResourceConverter
Added support for custom JsonConverters

I also have an example of JsonConverter for Day Entries. Some of these
might be able to get put into a NHarvestApi.Models and the existing
NHarvest code could be put into NHarvest.Core. Also, I'm seeing the
possibility of generalizing much of the core API code into a generic API
library...