Fixes for issues #370, #368 and #351

This is a possible patch for issue #370 - making it possible to customize the HTTP cache expiry header. 

I've added a new config property called httpExpiryInMinutes that is used by CachedCssResponse when setting the expiry header for a request. The default value is set to 1 week, which is what it was originally. 

The existing tests all pass, added some new tests to verify that the default value is still set to 1 week and that the new configuration value is picked up. 

I know this isn't the most pressing feature.. I just wanted to pick off something that looked fairly simple to get everything set up and to get to know the code a little bit. :)