Add RS Algorithm verification and refactor the way the JWTVerifier is inited.

Example usage:

For HS verification

String secret = "secret";
JWT jwt = JWTVerifier.init(HSAlgorithm.HS256, secret)
                     .verify("my.jwt.token");

Supported HS algorithms:
HSAlgorithm.HS256, HSAlgorithm.HS384, HSAlgorithm.HS512

For RS verification

PublicKey publicKey = parsePublicKeyFile();
JWT jwt = JWTVerifier.init(RSAlgorithm.RS256, publicKey)
                     .verify("my.jwt.token");

Supported RS algorithms:
RSAlgorithm.RS256, RSAlgorithm.RS384, RSAlgorithm.RS512