jwcrypto should use Exceptions with stack traces

[Austin King]

Currently, many error paths use a function and a string to indicate an error, instead of creating a new Error(msg) which would capture a stack trace.

Example:
"Not implemented: DS"
https://github.com/mozilla/jwcrypto/blob/master/lib/algs/index.js#L9

"malformed signature"
https://github.com/mozilla/jwcrypto/blob/master/lib/cert.js#L63

[Jared Hirsch]

Fixed by #89