Bugfixes and improvements

Hi.

I found this gem - thanks so much for the hard work.  I've added some extra functionality and done some bugfixes too. I've tried to group my commits into related changes. The commit log should have details, but here's a summary:

Copes with savings accounts. (previously would give a nil deference error)
Copes with (well, skips over) Scottish Widows investments. (same nil error)
Cope with zero balance (same nil error)
Can read further back than just 25 transactions.

I've started moving a big chunk of the transaction parsing into the Account class because that seemed cleaner to me than doing it in the Customer class. The only bit I'm not proud of is the massive chunk of code duplication - but I haven't yet thought of a clean way to read the initial 25 transactions because at that point the Account object hasn't been created.

What're your thoughts?

Thanks,

MattP