Add es_NI currency

Examples:

  python bin/num2words 1 -l es_NI --to currency
  un córdoba con cero centavos

  python bin/num2words 1.02 -l es_NI --to currency
  un córdoba con dos centavos

  python bin/num2words 2.02 -l es_NI --to currency
  dos córdobas con dos centavos

  python bin/num2words 1.01 -l es_NI --to currency
  un córdoba con un centavo

Signed-off-by: William José Moreno Reyes <williamjmorenor@gmail.com>

num2words/lang_ES_NI.py

@@ -0,0 +1,33 @@
+# -*- coding: utf-8 -*-
+# Copyright (c) 2003, Taro Ogawa.  All Rights Reserved.
+# Copyright (c) 2013, Savoir-faire Linux inc.  All Rights Reserved.
+
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+# Lesser General Public License for more details.
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+# MA 02110-1301 USA
+
+from __future__ import print_function, unicode_literals
+
+from .lang_ES import Num2Word_ES
+
+
+class Num2Word_ES_NI(Num2Word_ES):
+    CURRENCY_FORMS = {
+        'NIO': (('córdoba', 'córdobas'), ('centavo', 'centavos')),
+    }
+
+    def to_currency(self, val, currency='NIO', cents=True, separator=' con',
+                    adjective=False):
+        result = super(Num2Word_ES, self).to_currency(
+            val, currency=currency, cents=cents, separator=separator,
+            adjective=adjective)
+        return result.replace("uno", "un")