fix picture resource sync exception

[frostyplanet]

when syncing note with large picture, resource_ttype.data.body will be None, need to get download full resource with getResource()

traceback:

2015-07-23 23:31:08,608 - everpad-provider - DEBUG - Pulling note "美元指数" from remote server.
2015-07-23 23:31:11,280 - everpad-provider - ERROR - expected a character buffer object
Traceback (most recent call last):
  File "build/bdist.linux-x86_64/egg/everpad/provider/sync/agent.py", line 130, in perform
    self.remote_changes()
  File "build/bdist.linux-x86_64/egg/everpad/provider/sync/agent.py", line 170, in remote_changes
    note.PullNote(*self._get_sync_args()).pull()
  File "build/bdist.linux-x86_64/egg/everpad/provider/sync/note.py", line 181, in pull
    resource_ids = self._receive_resources(note, note_ttype)
  File "build/bdist.linux-x86_64/egg/everpad/provider/sync/note.py", line 287, in _receive_resources
    resource.from_api(resource_ttype)
  File "build/bdist.linux-x86_64/egg/everpad/provider/models.py", line 271, in from_api
    data.write(resource.data.body)
TypeError: expected a character buffer object
2015-07-23 23:31:11,281 - everpad-provider - DEBUG - Sync performed.