Implement web3.js registry integration

Should define a `provider` option to configure EthPM.js to connect to providers via web3.js.

Adhering to [registry service interface](https://ethpm.github.io/ethpm.js/interfaces/_ethpm_registries_.service.html).

Example implementation may be found in the [registry stub](https://ethpm.github.io/ethpm.js/modules/_test_stub_registries_.html)