Make Clock customization accessible for verification

[lbalmaceda]

Now when working with time related claims, the JWTVerifier allows to specify the custom Clock instance used on the verification. We want this to be as hidden as possible for the normal user. Devs can access it by performing a cast.

BaseVerification verification = (BaseVerification) JWT.require(Algorithm.RSA256(key))
    .acceptLeeway(1)
    .acceptExpiresAt(5);
Clock clock = new CustomClock(); //Must implement Clock interface
JWTVerifier verifier = verification.build(clock);

Related to: #50 #118

[lbalmaceda]

The last commit can be avoided, as it only extracts a single method interface for the Clock class. This can be set from a new constructor receiving the Date value to be returned in the clock.getToday() call.