Feed entropy into the RNG from mouse and key clicks

[tadasvaranavicius]

Current RNG does not gather enough entropy, making http://brainwallet.org/#tx insecure when reusing addresses.

https://github.com/bitcoinjs/bitcoinjs-lib/blob/master/src/jsbn/rng.js

// For best results, put code like // // in your main HTML document.

[brainwallet]

Merged, thanks a lot!