Client signatures

The client now prepends all script inputs with a signed message. A convenience function has been added to scripts.go to make generating a random signed message easier. Public/Private keys are stored in global variables. Error messages will be displayed if the user attempts to send a script input before generating a keypair.

`ResizeSectorErase()` was also fixed to use a uint16 for its atoms argument. This necessitated changing the way the associated opcode functions. Finally, the weight of each wallet is now printed during compile.