[Enhancement] Add package.json

[sudodoki]

Hi! It would be nice to add package.json file. Even if you don't publish this project to npm, people would be able to run npm install mtschirs/js-objectdetect which will enable easier usage with bundlers like browserify/webpack. Right now you would get an error of following sort:

npm ERR! eisdir EISDIR: illegal operation on a directory, read
npm ERR! eisdir This is most likely not a problem with npm itself
npm ERR! eisdir and is related to npm not being able to find a package.json in
npm ERR! eisdir a package you are trying to install.
npm verb exit [ -21, true ]