Dates are off by one in charts

[nylen]

The income & expense charts appear to have some issues with calculating dates. To reproduce on @redmountainmakers gnucash-django install:

1. View the PayPal account and filter for min date = 8/31/15 and max date = 9/1/15
2. Change the chart mode to monthly
3. Observe that the Income chart only shows income in August, which is incorrect (and that the Expenses chart doesn't even load)

cc @Velochic @StephenMRicks85 @jumijc