Added support for date ranges + expenses

I was going to have to download all my receipts from freeagent by hand - then I saw your code. Only took me a few minutes to set it up and add support for expense receipts (despite never having touched ruby before). You've saved me hours - so many many thanks.

I added support for expense receipts, and for specifying date ranges, which may (or may not) be of use to you.

thanks.