Addition of "start_interval" and "end_interval" root level keys to cruise.json

[DocOtak]

Future cruises may not have a known start and end date, but often a start and end period are known. This may also provide a solution to issue #13 if the actual dates are stored someplace separate.

The two interval keys would contain strings with ISO 8601 Duration strings in them.

{
  "startDate" : "2015-01-01",
  "start_interval" : "P2M"
}

The above example means that the actual start date of the cruise is sometime in the 2 month period after 2015-01-01. The duration notation is very expressive, see the wikipedia page for more information on it.

The new root level keys would be OPTIONAL.