core: Pass on the newly added box to the rest of the middleware stack
core: Scaffold an action for persisting box information
commands/box: Introduce a StateFile for keeping track of downloaded boxes
core: Write box information after download
commands/box: Extract box removal code from `box remove` command into a builtin action
core: Remove box information from state file after box removal
commands/box: Extract a base class for dealing with StateFile instantiation
commands/box: Preparing to display additional box information on `box list`
commands/box: List base box downloaded URL and datetime when `-i` gets provided to `box list`
core: Improve RemoveBoxInfo and WriteBoxInfo docs a bit
core: Get rid of code that deals with box info on a separate statefile
core: Persist box URL and download date into a JSON file under boxes directory
commands/box: Read box URL and downloaded date from JSON
Merge branch '2293-track-box-url' of https://github.com/fgrehm/vagrant into fgrehm-2293-track-box-url

Conflicts:
	plugins/commands/box/command/list.rb
commands/box/list: clean up