Add support for reading the cache response headers from Restify to set the EXPIRE in Redis

Additional response header X-Cache to indicate if the response was a cache HIT or MISS.  Tests and documentation are updated.
Change behavior of after to ignore cache if handling an error

For two reasons:
Because somewhere in the Restify execution chain, the rs.writeHead() method is invoked, thus locking the headers. If the after method were to fire, writing the X-Cache header will fail.
It seems unlikely that writing an error to cache is useful, especially if some middleware already passed an error into next or may have already wrote exception data to the response.
Fix the setting of X-Cache:Miss to the before function

The after function fires with the headers already locked (res.writeHead()) therefore and exception was thrown at runtime.
Merge pull request #13 from donaldjarmstrong/master

Respect Cache-Control headers