Improve packaging with package.json and makefile, also spelling etc

* Adds package.json. You can/should run npm publish to add this to
  npmjs.org so that people can use detect-zoom with browserify etc
* Adds a Makefile. Works with package.json. Users can build new
min versions by

npm install
make

Also fixes trailing space and spelling errors in readme.
Merge pull request #38 from tmcw/make-package

Improve packaging with package.json and makefile, also spelling etc