Add ability to use a local rpc provider

You can now use the SDK with a local blockchain (truffle / ganache). Instead of specifying a network, you can now pass your host.

// configure web3 with localhost
const web3 = bitski.getWeb3('http://localhost:9545');
// or just a provider
const provider = bitski.getProvider('http://localhost:9545');
const web3 = new Web3(provider);