Added a file to record last download datetime 

Sorry for pull request number 3, I've got my testing set up correctly now and it won't happen again.

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The file sits in the same location as the downloaded files. It is a
text file where the first line is a date time string specifying when
the last dataset was downloaded. The methods added are
get_local_metadata and set_local_metadata.

The prompt was also changed to give information to the user.