Bug 10401: Add ability to merge invoices

Given how easy it is to accidentally receive items from one invoice on
multiple invoices, the ability to merge invoices can be quite handy.
This patch adds that ability to Koha's Acquisitions module.

To test:
1) Apply patch.
2) Run unit test:
    > prove t/db_dependent/Acquisition/Invoices.t
3) Create two invoices from the same vendor for merging, and receive at
   least one order on each.
4) Do a search on the Invoices page that brings up both the invoices you
   created.
5) Check the boxes next to the two invoices.
6) Click "Merge selected invoices."
7) Choose which invoice you want to keep (the default will be the first).
8) Click "Merge."
9) Confirm that the resulting invoice has all the orders you received
   listed on it.
10) Sign off.