Allow setting credentials and certificate in callback ctor

This allows for a less verbose way of setting one-liners as these
callbacks.

pygit2/remote.py

@@ -78,6 +78,21 @@ class RemoteCallbacks(object):
     in your class, which you can then pass to the network operations.
     """
 
+    def __init__(self, credentials=None, certificate=None):
+        """Initialize some callbacks in-line
+
+        Use this constructor to provide credentials and certificate
+        callbacks in-line, instead of defining your own class for these ones.
+
+        You can e.g. also pass in one of the credential objects as 'credentials'
+        instead of creating a function which returns a hard-coded object.
+        """
+
+        if credentials is not None:
+            self.credentials = credentials
+        if certificate is not None:
+            self.certificate = certificate
+
     def sideband_progress(self, string):
         """Progress output callback
 

test/test_repository.py

@@ -514,13 +514,9 @@ class CloneRepositoryTest(utils.NoRepoTestCase):
         self.assertIsNotNone(repo.remotes["custom_remote"])
 
     def test_clone_with_credentials(self):
-        class MyCallbacks(pygit2.RemoteCallbacks):
-            def __init__(self):
-                self.credentials = pygit2.UserPass("libgit2", "libgit2")
-
         repo = clone_repository(
             "https://bitbucket.org/libgit2/testgitrepository.git",
-            self._temp_dir, callbacks=MyCallbacks())
+            self._temp_dir, callbacks=pygit2.RemoteCallbacks(credentials=pygit2.UserPass("libgit2", "libgit2")))
 
         self.assertFalse(repo.is_empty)