Not yet recommended to merge.

During tests an undocumented error (Office24Seven.Library.Economy.Accounting.AttachmentIsPostedException- Attachment is posted in the journal) was returned by 24/7 API that suggests different accounting years must be handled via separate stamp numbers (i.e. uploaded Attachments).

This can already be accomplished using the existing library by performing a duplicate upload (and acquiring its stamp number) for each year and performing separate ledger post requests.

It's possible that more information about the error or a change in 24/7 API constraints could allow this PR with its single-stamp-number / multiple bundle approach to succeed, after some further modification.

Here is a description of the current work and the limitation above:

• Each 24/7 API "bundle" can only specify a single accounting year, and their associated "voucher" must, too. See TFSO API docs
• Also, each 24/7 "attachment" (files upload) has its own stamp number. A request to save
  a "bundlelist" of "bundles" and their "vouchers" can only refer to just one attachment (not documented but encountered this error: Office24Seven.Library.Economy.Accounting.AttachmentIsPostedException- Attachment is posted in the journal)
• That restriction, in theory, shouldn't prevent two bundles, each with different year and appropriate voucher and entries in respective years, from being successfully processed, but that appears to be the case.
• So, this implies the entries of each accounting year must be posted with a different stamp number for each year, even though that means uploading the same files and creating a duplicate attachment for each year.