Support for TestRPC.provider()

I'd like to run my tests using an eth-lightwallet keystore and hooked-web3-provider.

This is working great with TestRPC if the transactions are sent over HTTP, but using `TestRPC.provider()` fails if I try to implement HookedWeb3provider (which seems to assume a HTTP `host`).

I was hoping to do something generic like this, that supports both HTTP hosts and `TestRPC.provider()`

```
// create the provider for web3 transactions using ks
const hookedProvider = new HookedWeb3Provider({
...web3.currentProvider,
transaction_signer: keystore,
});
// update the web3 instance to use our new provider
web3.setProvider(hookedProvider);
```

However, it fails with:

```
Error: Invalid JSON RPC response: Error: connect ECONNREFUSED 127.0.0.1:8545
```

Happy to help with a PR this is within the scope of hooked-web3-provider. Is it?