this mainly allows us to check if an account has a certain role and thus
if the account can call specific contract functions.
At some point we might want to extend that to support the check if an
account can call the function. For that we would need to have a mapping
between function names and roles, which we have not right now.
This moves the Kredits initialization to the instance which allows us to
be more flexible with handling contract addresses.
Example:
var k = new Kredits(provider, signer, {Registry: '0xabc'});
k.init().then((kredits) { ...});
var k = new Kredits(provider, signer, {Contributors: '0xabc'})
k.Contributor.add(...);
