fix(account):if new account can't pay NewAccountTax, it must be removed

When an account was deleted, it was not really deleted, its balance was just set to zero.

So the account was still considered as existing from the point of view of the System::account_exists() function.

Closes #60 (closed)

end2end-tests/cucumber-features/balance_transfer.feature

 Feature: Balance transfer
 
-  Scenario: If alice sends 5 ĞD to Dave, Dave will get 5 ĞD
-    Given alice have 10 ĞD
+  Scenario: Create a new account with enough founds
     When alice send 5 ĞD to dave
     Then dave should have 5 ĞD
+    When 1 block later
+    """
+    The blockchain should automatically withdraw account creation tax (3 ĞD)
+    """
+    Then dave should have 2 ĞD
+
+  Scenario: Create a new account without enough founds then retry with enough founds
+    When alice send 2 ĞD to eve
+    Then eve should have 2 ĞD
+    When 1 block later
+    """
+    The blockchain should automatically destroy Evec account
+    because Eve not have enough founds to pay the new account tax
+    """
+    Then eve should have 0 ĞD
+    When alice send 5 ĞD to eve
+    Then eve should have 5 ĞD
+    When 1 block later
+    """
+    The blockchain should automatically withdraw account creation tax (3 ĞD)
+    """
+    Then eve should have 2 ĞD
+