Rewrote the way filters work (initial version, work in progress).
You can learn more about the filters in the docstring of the
validate_filter method. Basically, you can apply the filters
defined there when invoking stamps:
stamps customer 2014-07-01
stamps customer 2014-07-01* -v
stamps customer 2w
Refactored and cleaned up some code (removed the duplicated details
method, details_by_customer)
Moved the print of the details (verbose) before the code that shows
the totals, so we can get a result that looks closer to what the
original stamp tool does.