Add `open` field to pygit's GitRepo model.

diff --git a/pygit/models.py b/pygit/models.py
index 7c4e519..db5d034 100644
--- a/pygit/models.py
+++ b/pygit/models.py
@@ -4,6 +4,7 @@ from django.utils.translation import gettext_lazy as _
 class GitRepo(models.Model):
   name = models.CharField(_("Name"),max_length=64)
   path = models.CharField(_("Path"),max_length=255)
+  open = models.BooleanField(_("Is open"))

   def __unicode__(self):
     return self.name