Idiomatic callbacks

This is certainly a case of pickiness, but `is_authorized?` and `is_conflict?` don't feel very idiomatic. I've renamed them to `authorized?` and `conflict?`, so that they more closely resemble, for example, `forbidden?`. I was also tempted to rename `generate_etag` to `etag`, but saw that a number of tests currently include `:etag`.

I realize that there would need to be a deprecation of the existing syntax, but I a) didn't know if you already have a deprecation policy or; b) whether you agreed with the changes to begin with.

Regards,

Jamie