Use zero to represent a nonexistent account

This also fixes a bug in the thin client where a nonexistent account
would have triggered a panic because we were using `balances[k]` instead
of `balances.get(key)`.

Fixes #534

src/request.rs

@@ -21,7 +21,7 @@ impl Request {
 
 #[derive(Serialize, Deserialize, Debug)]
 pub enum Response {
-    Balance { key: PublicKey, val: Option<i64> },
+    Balance { key: PublicKey, val: i64 },
     LastId { id: Hash },
     TransactionCount { transaction_count: u64 },
     SignatureStatus { signature_status: bool },