*
@method reset([unit] = 'hours')
@returns Date
@short Resets the unit passed and all smaller units. Default is "hours", effectively resetting the time.
@example

Date.create().reset('day')   -> Beginning of today
Date.create().reset('month') -> 1st of the month

*
function(unit) {
      var params = {}, recognized;
      unit = unit || 'hours';
      if(unit === 'date') unit = 'days';
      recognized = DateUnits.some(function(u) {
        return unit === u.unit || unit === u.unit + 's';
      });
      params[unit] = unit.match(/^days?/) ? 1 : 0;
      return recognized ? this.set(params, true) : this;
    }