Generalize access control mechanism

I've created a PoC for a more general access control mechanism for Arc, [this file](https://github.com/daostack/arc/blob/protected/contracts/controller/Protected.sol) contains a PoC with a simple usage example.
The basic idea is that there are a set of "locks"/"topics" each with a set of "keys". A contract can protects one of it's methods with a lock (or more) such that anyone with the appropriate key can use it to access this method.
Each key can have an expiration time, # of uses. Accounts can also transfer some (or all) of their capabilities to other accounts if allowed. Initially the contract itself has a "master key" (infinite uses, no expiration), and then it can pass some partial capabilities to the desired accounts.

Features:
- Can define a number of uses & expiration date on keys and if to enable the key to be transfrred.
- Accounts can transfer partial (or full) abillites to other accounts (if allowed).
- We can use advanced lock ids to lock a method even on specific parameters, enforce ordering, or other properties (see example).
- Can define complex boolean predicates to protect methods.
- No performance loss over hand-crafted mechanisms.

Benefits:
- Declarative policy (no more sprinkling `if`s all over the codebase)
- Easy control flow (ordering, timing, number of function calls) restrictions (e.g. only allowed to call `B` after calling `A` twice within 2 days).
- Allows accounts to call functions themselves instead of delegating through other contracts.
- Generalized logging mechanism: key usage events can be queried by clients to know whether a method was called and by which account, key revocations can be used to know when users no longer have access to a method.
- Easier security since we need to verify less code.