Adding command-line support for setting cache control metadata

as well as defaulting to `Cache-Control: private` when the `--private`
option is given.

Closes #4
Closes #5
Only doing the @public check once

since all files get the same cache control setting.  Also, the last
Travis build died during a `git clone` and I don't have "Restart Build"
rights...
Comparing ruby versions correctly to determine Array#sample/choice
Merge pull request #8 from meatballhat/cache-control-private

Adding command-line support for setting cache control metadata