Only one way to configure caching

The zen of python dictates that there should only be one way of doing
things, so I removed the cache decorator as it is not able to get
settings from the configuration.
Enable the Expires header as timedelta

In addition to the cache configuration that is defined while adding the
handler to the environment, you may now also set the Expires header in a
similar fashion:

    self.env.add_handler(..., expires=timedelta(minutes=10))
Caching docs
Release 0.4.0.beta