Make unconfirmed an i64

Some servers like electrs return the unconfirmed balance for an address as a
negative value.

Closes #45

Author: afilini

src/types.rs

@@ -208,7 +208,9 @@ pub struct GetBalanceRes {
     /// Confirmed balance in Satoshis for the address.
     pub confirmed: u64,
     /// Unconfirmed balance in Satoshis for the address.
-    pub unconfirmed: u64,
+    ///
+    /// Some servers (e.g. `electrs`) return this as a negative value.
+    pub unconfirmed: i64,
 }
 
 /// Response to a [`transaction_get_merkle`](../client/struct.Client.html#method.transaction_get_merkle) request.