This allows to filter contributors by the account entries.
For example:
```js
Contributor.filterByAccount({site: 'github.com'}); // returns all
contributors with github account
Contributor.filterByAccount({site: 'github.com', username: 'bumi'});
// returns bumi
```
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)
