Make package.json compatible for npm frontend use

[ThomasG77]

This PR augments package.json to ease OpenLayers 3 frontend development through NPM.
See this post on NPM blog to get more informations and evaluate if it's worth to accept.
If you use Browserify and/or Parcelify, you can use this informations out of the box.
browser part is in this spec

It's up to you to decide if providing paths to js and css is good.
In fact, it's quite similar to Bower but we don't create a new file, we only use existing package.json.

[tschaub]

Can you provide an example of something you can do after this change (but not before)?

[ThomasG77]

I've updated my PR. A bit ashamed...
You will see what the additional variables can do for you, looking at this small demo project
The README should be clear. Feel free to ask more.
I can also submit a new PR with only one change.

[elemoine]

I have no problem with that change. @tschaub?

[ahocevar]

https://www.npmjs.com/package/parcelify does not seem to be that popular. What do you think @tschaub?

[tschaub]

This looks ok by me. Thanks @ThomasG77.

[bartvde]

this breaks tools like dr-frankenstyle (https://www.npmjs.com/package/dr-frankenstyle used by Pivotal UI React) where style needs to be a string not an array:

> dr-frankenstyle css

TypeError: Arguments to path.resolve must be strings
    at Object.posix.resolve (path.js:439:13)
    at Object.callee$1$0$ (/Users/bartvandeneijnden/opengeo/git/sdk/node_modules/dr-frankenstyle-win/css-dependencies.js:81:39)
    at tryCatch (/Users/bartvandeneijnden/opengeo/git/sdk/node_modules/dr-frankenstyle-win/node_modules/babel-runtime/regenerator/runtime.js:67:40)
    at GeneratorFunctionPrototype.invoke [as _invoke] (/Users/bartvandeneijnden/opengeo/git/sdk/node_modules/dr-frankenstyle-win/node_modules/babel-runtime/regenerator/runtime.js:294:22)
    at GeneratorFunctionPrototype.prototype.(anonymous function) [as next] (/Users/bartvandeneijnden/opengeo/git/sdk/node_modules/dr-frankenstyle-win/node_modules/babel-runtime/regenerator/runtime.js:100:21)
    at GeneratorFunctionPrototype.invoke (/Users/bartvandeneijnden/opengeo/git/sdk/node_modules/dr-frankenstyle-win/node_modules/babel-runtime/regenerator/runtime.js:136:37)
    at lib$es6$promise$$internal$$tryCatch (/Users/bartvandeneijnden/opengeo/git/sdk/node_modules/es6-promise/dist/es6-promise.js:326:16)
    at lib$es6$promise$$internal$$invokeCallback (/Users/bartvandeneijnden/opengeo/git/sdk/node_modules/es6-promise/dist/es6-promise.js:338:17)
    at lib$es6$promise$$internal$$publish (/Users/bartvandeneijnden/opengeo/git/sdk/node_modules/es6-promise/dist/es6-promise.js:309:11)
    at lib$es6$promise$asap$$flush (/Users/bartvandeneijnden/opengeo/git/sdk/node_modules/es6-promise/dist/es6-promise.js:120:9)

[bartvde]

I guess I can whitelist the directories though to have it ignore the openlayers directory