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 to provide options like gas price/limit settings for the
state changing contract calls.
These options are simply passed to the ethers contract instance.
We need to provide the gas limit when using the jsonrpc provider.
(ganache failed with revert if not enought gas was provider)
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.
