Stages

This is a wrapper macro that supplies:

1. simple name-based memoization/checkpointing (note: not @memoize)
2. injected logger and ckpts arguments
3. invocation of stage functions is asynchronous via @spawn
4. results are returned as remote ref of tuple (result, logger-output)

Scripts using stages would ideally run in something like an ijulia notebook. Currently, ijulia notebooks don't handle magics, but eventually, when they do, we can clean up/manage checkpoints via filesystem commands.

Note: Ipython checkpoints provide script versioning but they don't checkpoint execution.

11/1 -- Last 0.3 version tagged

11/1 -- Initial port to 0.4 passing

7/27 -- ported to 0.6