Replace Clock contract instances by abstract Clock contract

[marcoonroad]

Related to #4. Creating a single and sole contract to just track one variable / attribute, without any encapsulated business logic and business invariants, is non-sense. It impacts our application with huge costs associated with the contract creation instruction (and its underlying gas consumption).

A better alternative would be to make the Clock contract abstract, and then clients inherit from it. In this way, the Clock contract's API / methods can be still preserved, somehow, we will just need to pass the sender's address into every call and make such methods private (they will be just called internally within the contract itself, this is to prevent malicious players from affecting internal clock / timing logic).