Add support for debug output that shows stage statistics

[nchammas]

Sometimes, especially if you are debugging something, you want to see a little more information on what Flintrock is doing.

One type of information I've often found myself wanting to see, for example, are statistics on how long it took to do the various sub-steps in an operation. How long is it taking to download Hadoop? How long does it take the Spark master to come up? And so forth.

So first we just need some minimal plumbing to support configurable debug output. From there we can add stuff like these statistics.

[nchammas]

As noted in #133, it may even be useful to see information like this by default.

[nchammas]

Retitling issue per this comment, though I'm thinking we could as well just roll this whole issue in #27.