enable jwt.ParsePublicKeyFromPEM to parse PKCS1 Public Key

[twocs]

#119 The code comment for jwt.ParsePublicKeyFromPEM stated that it could parse PKCS1 public keys. However, it couldn't. I created a unit test that generates a public PKCS1 key and the function cannot pass the test. I have added an extra attempt to parse the public key via the x509.ParsePKCS1PublicKey function, but also retained previous functionality that attempts to parse it from PKIX or certificate first.