README: JWT specs timestamps as seconds-since-epoch, but we use milliseconds

This documents our use of ms. Addresses PR #21.
log publicKey with .serialize(), not .toString()

.toString() produces "[object Object]" ...
more flexible parsing of public keys - handle case where it's the public-key property of an object
run jshint across source files
fix all jshint errors
remove dep on should, no need to introduce another assertion library dep
exact versions in package.json, proper falsey matching, correctly test if a parameter is a string
Merge pull request #71 from mozilla/jshint

add jshint tests, all code passing
Merge pull request #70 from mozilla/more-flexible-check-assertion

more flexible parsing of public keys in bin/check-assertion
Merge pull request #57 from jmandel/patch-1

README fix: log publicKey with .serialize(), not .toString()
Merge pull request #51 from mozilla/warner/timestamp-specs

README: JWT specs timestamps as seconds-since-epoch, but we use milliseconds
for expired certificates or assertions, specify which in error messages for simplified debugging
Merge pull request #74 from mozilla/better-error-messages

for expired certificates or assertions, specify which in error messages ...