Unsubscribe Fixes

Add an Unscubscribe model to the domain with properties as detailed in
the API docu. Update the Unsubscribe method not to expect an Email
instance back. Unsubscribe unit test now passes
Revert unintended change to MailChimp.Net.Tests.csproj

Revert unnecessary and unintended change to the Tests project file
Fix bug

Oops - bug - removed a variable, but left a reference
Merge pull request #7 from ja2/master

Unsubscribe Fixes merged to master