Configurable Cache Expiry (issue 370)

This is a fix for issue 370 - making it possible to customize the HTTP cache expiry header. 

I've added a new config property called httpExpiryInMinute 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.