Add an interface for JWTVerifier.

lib/src/main/java/com/auth0/jwt/JWTVerifier.java

@@ -14,7 +14,7 @@
  * The JWTVerifier class holds the verify method to assert that a given Token has not only a proper JWT format, but also it's signature matches.
  */
 @SuppressWarnings("WeakerAccess")
-public final class JWTVerifier {
+public final class JWTVerifier implements com.auth0.jwt.interfaces.JWTVerifier {
     private final Algorithm algorithm;
     final Map<String, Object> claims;
     private final Clock clock;
@@ -348,6 +348,7 @@ private void requireClaim(String name, Object value) {
      * @throws TokenExpiredException          if the token has expired.
      * @throws InvalidClaimException          if a claim contained a different value than the expected one.
      */
+    @Override
     public DecodedJWT verify(String token) throws JWTVerificationException {
         DecodedJWT jwt = JWT.decode(token);
         verifyAlgorithm(jwt, algorithm);

lib/src/main/java/com/auth0/jwt/interfaces/JWTVerifier.java

@@ -0,0 +1,16 @@
+package com.auth0.jwt.interfaces;
+
+import com.auth0.jwt.exceptions.JWTVerificationException;
+
+
+public interface JWTVerifier {
+
+  /**
+   * Performs the verification against the given Token
+   *
+   * @param token to verify.
+   * @return a verified and decoded JWT.
+   * @throws JWTVerificationException if any of the verification steps fail
+   */
+  DecodedJWT verify(String token) throws JWTVerificationException;
+}