Improve documentation and add a package.json file

[juliengirault]

Here a few suggestions:

• I think this plugin should be made available via npm.
  • Introduced package.json
  • Removed L.Map.mergeOptions() from the plugin itself.
    • My suggestion would be to give these options actual default values, so the developer only has to override what he wants?
  • Helps importing the plugin in local project.
  • Helps version tracking/upgrading.
• I documented the README config section a bit differently

[atstp]

Github's pr-timeline above is a bit confusing at the moment. Basically, the histories along with most of package.json was merged.

The readme definitely needs a brush-up, and while I'm not yet 100% sold on config changes there could be some solid arguments for it — i'll open an issue in just a minute.