However, some feedback:
It seems the time span involved could cause problems. Consider things like changing financial circumstances, expiring credit cards, closed accounts, deaths of backers, forgetful backers contesting charges, etc.
How will you prevent fraud? A dollar buys a vote, but the dollar goes to the winner... so just register two accounts and contribute enough to unilaterally decide that you yourself (on the other account) are the winner. Get your own money back plus everyone else's.
Similarly, what's to stop a backer from pledging some huge sum to get someone to develop something and then withdrawing their pledge at the last minute to get the benefit of the finished development without paying for it?
This isn't intended as my way of saying "you shouldn't do this", it's intended as my way of saying "don't get yourself into trouble by neglecting these issues."