This fixes the signature parsing and building to work correctly. Sometimes
a signature was one or two bytes too short, and needed to be padded. OpenSSL
can do this for us.
To avoid every user of the library needing to add and verify expiry info, this
is now handled by util.jwt itself (if not overridden or disabled).
Issuing tokens that are valid forever is bad practice and rarely desired, and
the default token lifetime is now 3600s (1 hour).
In many cases code will be either signing or verifying. With asymmetric
algorithms it's clearer and more efficient to just state that once, instead of
passing keys (and possibly other parameters) with every sign/verify call.
This also allows earlier validation of the key used.
The previous (HS256-only) sign/verify methods continue to be exposed for
backwards-compatibility.
