Conditional e-tags (#183)

Here are two extension methods for ETags and Last-Modified based on http://simoncropp.com/conditionalresponseswithnancyfx

```csharp
using Nancy.Caching;

public class SampleModule : NancyModule
{
    public SampleModule()
    {
        this.CheckForIfNonMatch();
        this.CheckForIfModifiedSince();

        Get["/img"] = x => Response.AsImage("pic.png");
    }
}
```

I added this extension method under `Nancy.Caching` namespace