Non-constant gas costs for token minting

https://github.com/ConsenSys/curationmarkets/blob/master/CurationMarkets.sol#L71

Implements a correct yet very expensive system for token minting that linearly increases in execution price with the token amount being bought.

I propose modeling this along the lines of what Bancor has done with their continuous price function.
(v. https://github.com/bancorprotocol/contracts/blob/master/solidity/contracts/BancorFormula.sol)
(v. https://drive.google.com/file/d/0B3HPNP-GDn7aRkVaV3dkVl9NS2M/view)