Added respect for cache-control max-age header

A cache-control response header with a max-age value will now override
any configured shelfLife by default.  Can be ignored by configuring
ignoreCacheControl to true

examples/single.coffee

@@ -7,5 +7,5 @@ test = ->
 		console.log "\t#{item.results[0].text}"
 		setTimeout(test, 1000)
 
-pantry.configure { shelfLife: 5, verbosity: 'DEBUG' }
+pantry.configure { shelfLife: 5, verbosity: 'DEBUG', ignoreCacheControl: true }
 test()

src/pantry.coffee

@@ -139,6 +139,10 @@ class StockedItem extends EventEmitter
 
 		request @options, (error, response, body) =>
 
+			# cache-control header max-age value will by default overide configured shelfLife
+			match = /max-age=(\d+)/.exec(response.headers['cache-control'])
+			@options.shelfLife = parseInt(match[1]) if match and not @options.ignoreCacheControl
+
 			unless error?
 				switch response.statusCode
 					when 304 # cached data is still good.  keep using it