Loom provider with optional private key

[eduardonunesp]

• LoomProvider constructor with option private key
• Add test for transactions and calls without private key

This PR makes possible to instantiate a LoomProvider without the needing of pass a private key on the second parameter

const client = createTestClient()
const loomProvider = new LoomProvider(client)

However, there's a case where the caller address should be passed, like:

const loomProvider = new LoomProvider(client)
const web3 = new Web3(loomProvider)
const contract = new web3.eth.Contract(ABI, contractAddress)

// Here we inform the from variable or the call will fail
const resultOfGet = await contract.methods.get().call({ from })

[eduardonunesp]

#291