Add code coverage thingy & get to 90%+

[taoeffect]

Problem

Without static type checking or a compiler, we need to test the heck out this thing!

Solution

Use flow and/or add code coverage generation to grunt test process.

• Set minimum threshold and tie it to Travis
• Display badge on README
• See this handy little bot: https://github.com/ethereum/go-ethereum/pull/2455#issuecomment-209975901
• Make sure it counts tests performed through Nightmare

[sandrina-p]

Same as #420, I don't see this to be a high priority for us to finish the Prototype.

We also have #632 saying the same thing and it has Low priority. My suggestion is:

1. update this issue with the additional info at Add code coverage to Cypress #632
2. close Add code coverage to Cypress #632 as duplicated.
3. Replace the priority label to Low.

What do you think @taoeffect?

[taoeffect]

I'm going to close this in favor of #632 and #528.