Add ERC20 tokens to etherscan.io sensor

[nkgilley]

Description:

Add ability to track ERC20 token balances to etherscan.io sensor.

You can provide a token symbol and ethereum address to fetch a, address' token balance. If the token symbol is not found, you can also provide the token's contract address and symbol manually.

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#4523

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: etherscan
    address: "0xfB6916095ca1df60bB79Ce92cE3Ea74c37c5d359"
  - platform: etherscan
    address: "0xfB6916095ca1df60bB79Ce92cE3Ea74c37c5d359"
    token: OMG
  - platform: etherscan
    address: "0xfB6916095ca1df60bB79Ce92cE3Ea74c37c5d359"
    token_address: "0xef68e7c694f40c8202821edf525de3782458639f"
    token: LRC

Checklist:

• The code change is tested and works locally.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.