Fixed auth problem if SKIP_AUTH was not defined
ResourceURI now written in logs when in DEBUG mode
Fixed an issue with Expires header (issue #12)

It seems it is required to call Slim expires() before etag()
because after calling etag() the app halts and the new expires()
value will not be written into the HTTP response headers.
Refactored HTTP cache headers calls

Model class now implements a setCacheHeaders() method that sets
both etag() and expires().
Extended setCacheHeaders() functionality (issue #12)

Extended the etag() unique-id generation by using two more salt
factors: VERSION constant defined in config file and GLOBAL_CACHE
_KEY defined in Model class. With the help of these two data
cached on client side can be more easily invalidated.