Add license file to npm package

[pblajer]

Background

It's a common practice to include license file in npm package/releases. MIT license states that it must be included in all copies of this software, so it forces users to download it manually (and releases themselves are not compliant with it).

Repro steps:

1. npm install jsrsasign
2. ls ./node_modules/jsrsasign

Current:

No license files to be found in listed files

Expected:

License file is listed