Rework ETag feature

[lafrech]

Extract ETag feature in a separate mixin to decouple it from response marshmaling.

Main changes:

• Etag is now managed from a new @etag decorator.
• check_etag and set_etag are methods of EtagMixin, therefore of Blueprint.
• No need to use disable_etag_for_request: just don't use @etag decorator.
• List of headers to include in ETag computation is configurable in Blueprint.ETAG_INCLUDE_HEADERS.

TODO:

• Changelog
• Docs
• Replace ETAG_ENABLED config variable with ETAG_DISABLED (à la flask-login)?

[coveralls]

Coverage increased (+0.2%) to 99.82% when pulling 3652561 on dev_rework_response_etag into 304bc0f on master.