Adds ability to use a custom date format

A date format string can be entered as the prop `dateFormat` to the `DatePicker` element. If you do not enter this prop, it will default to `YYYY-MM-DD`, which was previously the only valid format allowed. The requirements of one of my current projects was that I needed a different format, so I put in the issue https://github.com/Hacker0x01/react-datepicker/issues/45. Please let me know if you have any questions. Thanks!
Let's default to `YYYY-MM-DD` by setting it in `getDefaultProps`.