Switch from mocha to micro-should for lightweight testing

[paulmillr]

I've did this in branch https://github.com/ethereum/js-ethereum-cryptography/tree/m-should, but can't remember why it has not been merged.

micro-should is simple 200-line alternative to mocha. I think it should work in all browsers, but unclear

[paulmillr]

OK it requires writing a custom adapter to karma, for testing in browsers. Not great.