Save Map function doesn't work with UnicodeEncodeError.

I get follow error when click "Save Map" in "Create a New Map" view.
```
Internal Server Error: /maps/new/data
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py", line 112, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/python2.7/dist-packages/geonode/maps/views.py", line 406, in new_map_json
    map_obj.save()
  File "/usr/local/lib/python2.7/dist-packages/polymorphic/polymorphic_model.py", line 89, in save
    return super(PolymorphicModel, self).save(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/db/models/base.py", line 545, in save
    force_update=force_update, update_fields=update_fields)
  File "/usr/lib/python2.7/dist-packages/django/db/models/base.py", line 582, in save_base
    update_fields=update_fields, raw=raw, using=using)
  File "/usr/lib/python2.7/dist-packages/django/dispatch/dispatcher.py", line 185, in send
    response = receiver(signal=self, sender=sender, **named)
  File "/usr/local/lib/python2.7/dist-packages/geonode/social/signals.py", line 111, in activity_post_modify_object
    verb='{verb}'.format(verb=str(verb)),
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-3: ordinal not in range(128)
```
I fix this error.