text: Title: append not working as expected
  Body: I have a dataframe stored in mongodb using arctic and I would like to append to the existing dataframe, e.g. updating daily prices.
  I've tried using version storage and the append() function, however it gives me not implemented for handler error
  
  " File "C:\Anaconda\lib\site-packages\arctic\store\version_store.py", line 496, in append
      raise Exception("Append not implemented for handler %s" % handler)
  Exception: Append not implemented for handler <arctic.store._pickle_store.PickleStore object at 0x09274AB0>"
  
  've tried register_library_type('dataframestore', PandasDataFrameStore) but received some other error.
  
  Do you have an example of how to update existing dataframe/series data or is there a rule of thumb?
  