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(...);
This allows use to call the methods with a string but still store the
name as bytes32.
I also had some issues when calling the methods from ethers.js and a
string parameter so this is easier and safer.
