Node support update

[AdriVanHoudt]

• Add testing on node 4 and 6, drop support for 0.10 and 0.12.
  see https://s3.amazonaws.com/heroku-devcenter-files/article-images/703-imported-1453533598-schedule.png for info on the LTS of the versions
• Make use of new node 4 features if it makes sense

[AdriVanHoudt]

closed in #21