Merge pull request #15 from digitalbazaar/master

Fix typos.
fix test command in package.json
Merge pull request #19 from maxogden/testfix

fix test command in package.json
Remove typo.
Fix test signatures.
Add tests.

- Add invalid date test.
- Add simple sign/verify test.
- Use fixed past date headers.
- Ignore clock skew so a fixed past date can be used to check signature
  output.
- Check Authorization header results.