Enforce list type for nodes

[cyon1c]

IE - if nodes is a string

Closes #156

[roadscape]

Is it exceedingly unlikely that this string might have comma-separated URLs?

[cyon1c]

Excellent point. We should:

• Determine all valid inputs
• Check for and sanitize valid inputs.
• Throw errors on invalid input explicitly.

I propose the formats we would accept:

• [x] list of strings (already working)
• [x] single node string (as of PR)
• [x] string of comma separated nodes

In addition:

• [x] In http_client.py add a sanitize_nodes method in init before cycle(nodes) is called.