Merge pull request #30 from techtonik/master

Enable strict mode for Springy.js
Merge remote-tracking branch 'techtonik/master'

* techtonik/master:
  Revert mixed argument patch for addNode(), because addNodes() in loadJSON already supports single string argument.
  Add JSON load capabilities to Springy.
  API change: addNode() now accepts string or Node
  Make demo2.html output different from demo.html