Save checkpoints as a diff  #4

The current checkpoint strategy is crude and stores a full copy of each checkpoint data. This is wasteful for large objects.

It would be nice to use some diff format to save the differences between checkpoints. Here are a some possible libraries that could be used (I haven't spent any time researching these yet):

• http://jsonpatch.com/
• http://tlrobinson.net/projects/javascript-fun/jsondiff/
• http://benjamine.github.io/jsondiffpatch/demo/index.html
• http://json-delta.readthedocs.org/en/latest/
• http://stackoverflow.com/questions/8431651/getting-a-diff-of-two-json-objects
• https://github.com/benjamine/JsonDiffPatch
• https://www.npmjs.org/package/json-diff