Simple way to add a Cache-Control header, fixes #71

This adds a new method stubbles\webapp\response\Headers::cacheControl()
which allows to add a Cache-Control header to the response via
stubbles\webapp\response\Response::headers(). The default value of the
header will contain the "private" directive only, but this can be
changed by enabling the various directives on the
stubbles\webapp\response\CacheControl instance returned by
stubbles\webapp\response\Headers::cacheControl().

The implementation follows http://tools.ietf.org/html/rfc7234, any
wrong outcomes may be due to wrong understanding of the RFC and may be
considered a bug.