Allow the user to specify the account that a saldo belongs to.

app/views/saldos/_form.rhtml

 <%= error_messages_for 'saldo' %>
 
 <!--[form:saldo]-->
+<p><label for="saldo_account_id">Account</label><br/>
+<%= select "saldo", "account_id", Account.find_all.collect { |a| [a.name, a.id] } %>
+
 <p><label for="saldo_amount">Amount</label><br/>
 <%= text_field 'saldo', 'amount'  %></p>