Add a command to API to create Proof Of Work

Since using JS it would be too slow, and an attacker could always make native code to spam a site and compute a POW much faster than people generate them using JS. And it would allow to have one "right" implementation, instead of having multiple implementations, some which could even be failing to do their job correctly, it would provide a `generatePOW` with a difficulty parameter, and a `verifyPOW` API commands. It could be added to ZeroMail and to ZeroTalk, it would make spamming much harder.