remove "padding days" hack

[Zecat]

Assuming days are in a container with --layout-wrap according to #104, the "empty days" can be avoid by adding to months a field "firstDay" you get here https://github.com/bendavis78/paper-date-picker/blob/master/paper-calendar.html#L339, and put before the dom-repeat of days:

<div style$="width: calc(14.285% * [[month.firstDay]])"></div>

100 / 7 ~= 14.285
It will shift the first day at the right place.

[bendavis78]

Does the current method of padding days cause any issues, performance or otherwise?

[Zecat]

It will lighten '_month' array and so avoid some dom-repeat iterations to finally lighten the dom a little bit, and it makes more sense that theses days does not exist rather than make them disabled, just an improvement.