Add port of RSABigInteger from JavaScript

Add port of RSABigInteger from JavaScript: username1565/BigInteger.js@2b2057d
This code need to test, optimize, and remove the redundant bullshit.
All this need to encrypt (by pub)/decrypt (by priv), and sign (by priv)/verify (by pub) the messages, as bytearrays with arbitrary bytelength.
See diff for this commit, and comments, inside this.