Add support for ETag

[levkk]

Add support for ETag in the StaticFiles controller.

[siddahrta-max]

This should be done now. If-None-Match with a single ETag or a list of ETags as well as If-Modified-Since is now supported.
The StaticFiles struct has a method to replace the generation functkion (per Controller) so one Controller can generate e.g. MD5 and another SHA1 Hashes to create ETag Header.