Make time and offset arguments to Signature optional

If these are left blank, use git_signature_now to insert the current time.
Specifying one but not the other causes a TypeError.
Test inlcluded.

src/pygit2/signature.c

@@ -49,7 +49,7 @@ Signature_init(Signature *self, PyObject *args, PyObject *kwds)
         return -1;
     }
 
-    if (!PyArg_ParseTuple(args, "OsLi|s",
+    if (!PyArg_ParseTuple(args, "Os|Lis",
                           &py_name, &email, &time, &offset, &encoding))
         return -1;
 
@@ -57,7 +57,14 @@ Signature_init(Signature *self, PyObject *args, PyObject *kwds)
     if (name == NULL)
         return -1;
 
-    err = git_signature_new(&signature, name, email, time, offset);
+    if (PySequence_Length(args) == 2) {
+        err = git_signature_now(&signature, name, email);
+    } else if (PySequence_Length(args) == 3) {
+        PyErr_SetString(PyExc_TypeError, "offset must be specified with time");
+        return -1;
+    } else {
+        err = git_signature_new(&signature, name, email, time, offset);
+    }
     free(name);
     if (err < 0) {
         Error_set(err);

test/test_signature.py

@@ -28,6 +28,7 @@
 from __future__ import absolute_import
 from __future__ import unicode_literals
 import unittest
+import time
 
 from pygit2 import Signature
 from .utils import NoRepoTestCase
@@ -50,6 +51,17 @@ class SignatureTest(NoRepoTestCase):
         self.assertEqual(signature.name, signature._name.decode(encoding))
         self.assertEqual(signature.name.encode(encoding), signature._name)
 
+    def test_now(self):
+        self.assertRaises(TypeError, Signature, 'Foo Ibáñez',
+                          'foo@example.com', 1322174594)
+        signature = Signature('Foo Ibáñez', 'foo@example.com')
+        encoding = signature._encoding
+        self.assertEqual(encoding, 'utf-8')
+        self.assertEqual(encoding, signature._encoding)
+        self.assertEqual(signature.name, signature._name.decode(encoding))
+        self.assertEqual(signature.name.encode(encoding), signature._name)
+        self.assertTrue(abs(signature.time - time.time()) < 5)
+
 
 if __name__ == '__main__':
     unittest.main()