Add Cacheable extension to allow Cache-Control header

Create a NancyFx extension called Cacheable, which adds the Cache-Control header (to Get/Head requests only) to Responses. Enabled it for a bunch of methods where the responses are fairly static.

Per conversation with Saganaki on creating the Cacheable extension to NancyFx, we're adding a NancyExtensionMethod class. It's apparently more C#-ish. This will give us a spot to put some more NancyFx extensions as we continue to build.