On another note, it is 12 days before the election, and over 16 million votes have already been cast, which is likely to be a little more than 10% of the total.
http://www.electproject.org/early_2016
Importantly, it's not evenly distributed, and there are some key states where the early vote is closing in on 25% or even higher:
(numbers are approximate because I'm lazy)
Arizona: 882,000 votes cast, a bit less than 40% of the 2012 total;
Colorado:575,000 votes cast, or about 20% of the 2012 total;
Florida: 2.5 million votes cast, or a little more than 30% of the 2012 total;
Georgia: about 1 million votes are in, or a bit more than 25% of the 2012 total;
Iowa: almost 350,000 votes in, or about 20% of the 2012 total;
Nevada: 280,000, or 28% of the 2012 vote; and
North Carolina: 880,000, or almost 20% of the 2012 total.
There are some exceptions, but it does appear to me that the "early vote" is disproportionately concentrated in the most competitive swing states. One notable exception s Pennsylvania, which doesn't have early voting.
One thing to keep in minds as polls "move" closer to Election Day is that the cake is increasingly more "baked" as that day gets closer. I'm not one who thinks we can learn much about who is "winning" based on early voting numbers--but I do think it's safe to conclude that as Election Day gets closer it is harder to move the needle when a significant part of the vote is already set in stone.