Define storage collateral function

#60 was closed without actually defining storage collateral anywhere.

- What is the function for storage collateral requirements? Is storage collateral a fixed amount per byte? Or, like pledge collateral, should it scale with FIL in circulation so as to represent a fixed fraction of the total market value.
- Is storage collateral represented in actor state anywhere, or always computed as a function of committed sector count*size.
- The CommitSector actor method must check that the miner has sufficient storage (as well as pledge) collateral.

As always, implementers don't need commitment to specific constants, but we do need to know what it's a function of and whether/how it's stored in actor state.