use es6 module syntax

[sualko]

This is a first proposal to fix #63. There are still some todos:

• fix intention
• use npm packages if possible (requires rollup-plugin-node-resolve to create a browser bundle)
• ww tests fail for unknown reason
• npm prepublish hook to create bundles
• maybe move all export statements to the end of a file

What do you think?

[sualko]

@arlolra can you look into this?

[arlolra]

I'm currently travelling but hopefully soon.