exclude test folder from package

[dbrugger]

Resolves #18

Dry run:

> npm pack
npm notice
npm notice 📦  [email protected]
npm notice === Tarball Contents ===
npm notice 899B  factor.js
npm notice 5.3kB index.js
npm notice 792B  bin/pem-jwk.js
npm notice 634B  package.json
npm notice 500B  README.md
npm notice === Tarball Details ===
npm notice name:          pem-jwk
npm notice version:       2.0.0
npm notice filename:      pem-jwk-2.0.0.tgz
npm notice package size:  2.9 kB
npm notice unpacked size: 8.2 kB
npm notice shasum:        f032fffbc6d0d50989d294230fa354cc75b2d60e
npm notice integrity:     sha512-GAQsQ6kBt72Lv[...]0dJZjLcjILyDQ==
npm notice total files:   5
npm notice
pem-jwk-2.0.0.tgz