Refactored to use ServiceStack to create URL Request

Previously the URL was created using ReturnFullUrlWithParameters, which
was a custom function.

This was rewritten to use ServiceStack, which allowed the custom
ReturnFullUrlWithParameters function to be eliminated.
Added tests for MailChimpExportManager
Merge pull request #93 from Richard-West/master

Refactored MailChimpExport.cs to use ServiceStack to create URL Request