Minor refactoring of QuandlDotNet class

Changes:

* Removed unused OutputFormat variable
* Used StringBuilder to construct the request Url.
* Replaced Activator.CreateInstance with a tweaked version based on  http://rogeralsing.com/2008/02/28/linq-expressions-creating-objects/
* Changed GetData and GetRawData to accept/return Interfaces instead of concrete implementation of IList, IDictionary
* Removed unused references