Use `normalizeAssertion` in the issuer endpoint

We want a normalized assertion structure to pass to awardBadge, so we
shouldn't just directly use the `assertion` property from the
`analyzeAssertion` return value.
Support new assertions in `lib/award.js`

`award` now accepts a `signature` property and won't try to bake a badge
that doesn't have an endpoint set.

We need to figure out badge baking for signed badges, but for now we
don't have time to deal with it.