added support for RSA signatures.

I've added support for RSA signatures. 

Usage : 

```
/* sign */ 
var signatory = new JSEncrypt();
var privateKey = "RSA_PRIVATE_KEY";
signatory.setPrivateKey(privateKey);
var message = "I'm a message, sign me !";
var rsa_sha256_signature = signatory.sign(message, CryptoJS.SHA256);

/* verify */
var verifier = new JSEncrypt();
var publicKey = "RSA_PUBLIC_KEY";
verifier.setPublicKey(publicKey);
if (verifier.verify(message, rsa_sha256_signature, CryptoJS.SHA256)) {
 console.log("valid signature !");
}
```