Adding support for Elliptic Curve signatures (ES256 / ES384 / ES512)

This PR adds support for the ES256, ES384, and ES512 JWT signature standards by utilizing the pure-Python `ecdsa` library.
I had to increase the expiration time in order to get the tests to finish before the expiration time would breach.  On my local system, 3 seconds was sufficient, but I wanted to ensure that less powerful systems would be able to also pass the tests.