Bonded Stealth TX

> name is not final, all suggestions are welcome

$3

- keeper calls bond with msg.value
- keeper calls contract with a random _stealthHash
- contract uses validateStealthTx modifier to validateHash with msg.sender, hash and penalty
- if valid (no one reported it) execution continues.
- if invalid (hash was reported) msg.sender loses it's bond and execution stops.

Improvements:

- update README (how does it work)

- tests

- check remaining gas on validate hash to avoid gasLimit-error exploit to avoid getting penalized