Add sign/verify support

[Spark-NF]

Two methods were added to the JSEncrypt class:

• sign, which takes a string to sign, a hash method, as well as the name of the hash method
• verify, which takes a string to verify, its signature, and a hash method

The name of the hash method is required because RSA signatures need a header depending on which hash was used (see https://tools.ietf.org/html/rfc3447#page-43).

I tested that generated signatures can be verified by JSEncrypt, jsrsasign, and SubtleCrypto with no issues. It can verify signatures from JSEncrypt, jsrsasign, and SubtleCrypto also.

Fixes issue #2.

Signing example

var data = "some data";
var jse = new JSEncrypt();
jse.setPrivateKey(privateKey);
var signature = jse.sign(data, CryptoJS.SHA256, "sha256");

Verifying example

var data = "some data";
var jse = new JSEncrypt();
jse.setPublicKey(publicKey);
if (jse.verify(data, signature, CryptoJS.SHA256)) {
    console.log("Valid signature.");
}

[harpcio]

+1

[travist]

Well done.

[travist]

Do you mind adding some docs to the Readme.md file for this?