+ http: add model for conditional headers
`If-(None-)-Match`
`If-(Un)Modified-Since`
ETag
`Last-Modified`
+ routing: add `conditional` directive
+ routing: add `conditional` support to FileAndResourceDirectives
+ http: add model for `If-Range` header
= routing: extend `conditional` directive to also support `If-Range` header (test still missing)
= routing: minor fixes for FileAndResourceDirectives
! http, routing: minor refactoring of ETag matching
Merge pull request #797 from 2beaucoup/caching

Enable client caching (supercedes #776)