A first attempt at validator.getAssertionGUID().

Still need to document how it works in the README, but for now you
can look at the tests.
Changed getAssertionGUID() algorithm to output SHA256 hashes, not URLs.

@brianloveswords argued that the arbitrary signed-assertion protocol
has no way of actually prescribing a location that can be found
somewhere, which is true. So instead of representing the GUID as a URL,
we will represent it as a hex-encoded SHA256 hash.

If the assertion is hosted, the GUID will be:

  sha256('hosted:' + assertion.verify.url)

Whereas if the assertion is signed, the GUID will be:

  sha256('signed:' + assertion.uid + ':' + origin(assertion.verify.url))
Documented validator.getAssertionGUID() in README.md.
added example to end of README.
Merge branch 'master' into get-assertion-guid

Conflicts:
	index.js
Merge pull request #9 from toolness/get-assertion-guid

Added validator.getAssertionGUID().
clarified what 'validator' in the usage section of the readme refers to.