Add node health info

Closes #3 (closed).

@brianflanagan I removed all the text on the main page, and condensed it into a small indicator, with a tooltip for details.

The indicator has 3 states:

• OK: everything is normal
• NEEDSATTENTION: happens when connectivity is weak, or when client is not sync
• BAD: something is wrong, can't connect with parity

The tooltip has these details:

• OK: number of peers
• NEEDSATTENTION: if not sync: it shows the sync progress. if poor connectivity: shows a small message.
• BAD: a message describing the problem.