add support for date lookups

[timgraham]

MongoDB Engine implemented year lookups by querying by date range. That does not work for month and day lookups (those were unsupported). Regex matching could be used for year, month, and day, but nothing more advanced like day of week.

I think the $expr operator offers the greatest capability, e.g.

day = 0
db.mydatabase.mycollection.find({ 
    "$expr": { 
        "$eq": [ { "$dayOfWeek": "$date" }, day ] 
    } 
})

[timgraham]

These queries don't work (without using the $dateFromString operator) unless datetimes are stored as JSON Date objects. This will probably done as part of #11.