Deprecate and rename the accept_untrusted_content=False keyword

[rneilson]

The accept_untrusted_content=False keyword in Receiver() and Sender.receive() is somewhat misleading, and has caused some confusion about its intended usage (see discussion in #24, #40, and #43).

I see two possibilities:

1. Rename it to always_hash_content=True, to mirror the same keyword argument in Sender() and Receiver.respond().

2. Rename it to require_content_hash=True, which is perhaps the more accurate terminology, as it allows requests to omit the hash attribute in the Hawk header, but still validates the content hash if included.

[kumar303]

whoops, I missed this. You should be able to assign me PRs that need review now. I'll assign myself.

[kumar303]

Hah, github fail. I thought it was a PR, nevermind.

[kumar303]

I think require_content_hash=True makes the most sense in the context of a receiver, let's go with that.

[rneilson]

Cool, will do a PR for that. Any particular way you'd like me to mark the deprecation? Just log.warning?

[kumar303]

Hmm, I forget how to do this. log.warning is probably fine. It should have test coverage to make sure that the warning gets logged. I seem to recall there is another way to do deprecations but I'm not sure if it's helpful or not since the logging channel may be different.

[rneilson]

Alright. Give me a day or two, and I'll shoot you a PR.