Implement fetching and saving CSVs

Hi,

I was really pleased to find that someone had already done the bulk of the work on this! This branch let me fetch a bunch of CSVs by running:

    python download.py -u MY_LLOYDS_TSB_ID 2011/01/01--2011/03/31 2010/10/01--2010/12/31 2010/07/01--2010/09/30

This is not very robust or well-tested (and I note that urllib2 and hence mechanize do not check SSL certificates! Danger!) but it does work for me. It would obviously be better to automate breaking down the range into smaller chunks until there are less than 150 transactions, but … this was good enough for me today!