Move Account into its own module

Also use default Default generator, since system program ID is
[0; 32]. Bank should probably be the one to set this anyway.

src/rpc.rs

@@ -1,6 +1,7 @@
 //! The `rpc` module implements the Solana RPC interface.
 
-use bank::{Account, Bank, BankError};
+use account::Account;
+use bank::{Bank, BankError};
 use bincode::deserialize;
 use bs58;
 use jsonrpc_core::*;