Datepicker v-model should return Date, not string

[xkjyeah]

The current version is unworkable, because

• we cannot parse the date without knowing the format
• human readable formats (in Western cultures) are not computer-friendly (e.g. d-MMM-yy is not sortable, nor are they JSON serializable)

[bezborodow]

Any further thoughts on this? I have encountered the same problem trying to bind to a date field. See also #88.