Save all the blocks!

- Bringing in LevelDB to store blocks and transactions (hopefully not a
permanent thing, because this will make the resulting binaries
unnecessarily platform-specific).
- Storing each block with its transactions as we read it in.
- Verifying scripts in all transactions before marking a block valid.
--- ...except for the signature check, which is the most important part.
Right now, it's just hand-waving and returning "true" for those checks.
But the rest of the script still has to be valid!