clean up Makefile and add 'make cutarelease' to publish new versions

[trentm]

• [x] drop unneeded goop from makefile
• [x] add 'make cutarelease' based on same from joyent/node-manta (among others)
• [x] use "files" in package.json to reduce the extra stuff included in the published package

[trentm]

Before "files":

$ npm pack
npm notice
npm notice 📦  http-signature@1.3.1
npm notice === Tarball Contents ===
npm notice 896B   package.json
npm notice 178B   .dir-locals.el
npm notice 1.7kB  CHANGES.md
npm notice 13.8kB http_signing.md
npm notice 2.4kB  Jenkinsfile
npm notice 1.1kB  LICENSE
npm notice 1.6kB  README.md
npm notice 626B   lib/index.js
npm notice 10.1kB lib/parser.js
npm notice 13.0kB lib/signer.js
npm notice 2.9kB  lib/utils.js
npm notice 3.1kB  lib/verify.js
npm notice === Tarball Details ===
npm notice name:          http-signature
npm notice version:       1.3.1
npm notice filename:      http-signature-1.3.1.tgz
npm notice package size:  15.7 kB
npm notice unpacked size: 51.4 kB

after "files":

$ npm pack
npm notice
npm notice 📦  http-signature@1.3.1
npm notice === Tarball Contents ===
npm notice 924B   package.json
npm notice 1.7kB  CHANGES.md
npm notice 1.1kB  LICENSE
npm notice 1.6kB  README.md
npm notice 626B   lib/index.js
npm notice 10.1kB lib/parser.js
npm notice 13.0kB lib/signer.js
npm notice 2.9kB  lib/utils.js
npm notice 3.1kB  lib/verify.js
npm notice === Tarball Details ===
npm notice name:          http-signature
npm notice version:       1.3.1
npm notice filename:      http-signature-1.3.1.tgz
npm notice package size:  9.9 kB
npm notice unpacked size: 35.0 kB