Allow the PMI URL to directly contain the PMM

[domenic]

Closes #10. In particular, this switches from a HEAD request to a GET request, so that we can inspect the entire body if the Link header is not present.

@rsolomakhin to review.

[rsolomakhin]

It's a bit hard to follow the respec formatting. Just to double-check: the spec now issues a GET request and looks for Link first. If Link is found, GET that, otherwise return the contents of the first GET. Is my understanding correct?

[domenic]

Yeah, that's it exactly. The body of the response gets thrown away if any Link is successfully found. I also threw in the ability to terminate the GET if you find such a Link.

Note that unparseable Link values will, in this spec, fall back to trying the body.

[rsolomakhin]

Note that unparseable Link values will, in this spec, fall back to trying the body.

Nice touch!