The RSA Key Encapsulation Mechanism (RSA-KEM) from ISO 18033-2.

Add implementation of RSA Key Encapsulation Mechanism. 

Add implementation in
- js/kem.js
- test/kem.js

Fix mgf1.js to can make hash with other size than the digest (the only way to implement test vectors)

At any doubt please contact me to lautaro.cozzani@scytl.com or lautaromail@gmail.com