AA Battery Box

I redesigned this Battery box starting from the excellent job published by [Peterpur](http://www.thingiverse.com/thing:162966).
I did my new release of the battery box to improve two aspects:
1. applied round upper and lower corners as they resulted a bit too sharp;
2. applied a guide for rubber band to keep box closed even if carried in a mountain bike backpack.
I also modified external shape to follow batteries body, but it is just an aesthetic improvement with no other purposes.

Update of Sept. the 26th
I modified the box a little:
- modified side holes to elliptical; it will help on avoid overhanging when printing upper side;
- enlarged battery slots to have more internal clearance.

Cover and external dimension of the box remain unchanged.

I suggest to use the box puttin batteries with "+" sign towards the cover whether they are full; with "-" towards bottom when they need recharging.
This will help managing batteries.