implemented ApiKeyAuth authorization filter

Implemented an API key authorization filter. More info on `ApiKeyAuthAttribute`:

[API Key Authorization Through Query String In ASP.NET Web API AuthorizationFilterAttribute](http://www.tugberkugurlu.com/archive/api-key-authorization-through-query-string-in-asp-net-web-api-authorizationfilterattribute)

I would also add a sample application which demonstrates the usage of this but couldn't figure out how you guys manage the sample apps inside the repo.