New statistics endpoint for invoices, and a new account statement endpoint

We have introduced two new endpoints:

• GET /statistics/invoices — aggregated statistics for your invoices and credit notes, e.g. spend by category, and net/VAT/gross totals.
• GET /user_ledger_entries — your account statement: individual balance movements (refills, charges) with a running balance.

See the API reference for /statistics/invoices, and here for /user_ledger_entries.