Code update.

1.  Added progress messages to existing make scripts.
Code and documentation update.

1.  Modified GET_DATA_GOOGLE so that it returns eight columns of data
    rather than six. The newly-added 'Dividend' and 'Split' columns are
    filled with NaNs as Google Finance does not currently make its
    dividend or split data available for download.
2.  Modified TEST_GET_DATA_GOOGLE to cope with the presence of NaNs in
    the test data.
3.  Added a comment to GET_DATA_YAHOO and GET_DATA_QUANDL that explains
    the presence of any NaNs in their return data.