Create timezone-aware datetime objects for issue #317.

Need to document that when entering competiton's end dates and competition phase's start dates in the YAML file, the valid formats are:

- `YYYY-MM-DD`

    For example, `end_date: 2014-03-01` specifies a competition end date of March 1st, 2014 at midnight UTC.

- `YYYY-MM-DD HH-MM`

    `start_date: 2014-03-01 18:15` specifes a start date of March 1st, 2014 at 6:15pm UTC.