New receive screen (ready, please review)

When the received amounts differ from those ordered, redistribution of changed quantities over members is manual. This pull request adds a "receive" screen in the orders section, where amounts received can be entered, after which products are redistributed over members using the existing algorithm.

Includes the addition of a `units_received` field in `OrderArticle`. An additional `units_billed` field is created as well although it has no user-interface yet (but this is very much similar to this screen).

Where `OrderArticle#units_to_order` was used, you'll probably want to use `OrderArticle#units`, which takes the last entered amount (ordered, billed, received - in that order). Where the amount of units of closed orders is shown, there now is a title attribute showing the history (e.g: "2 ordered, 1 billed, 1 received").

A new convention is introduced: whenever a number is the amount of wholesale units (full boxes, unit_quantities, lot, whatever), a watermark image of a box is shown next to it. I hope this brings clarity to whether numbers are member units or wholesale units.

See also foodcoop-adam#2 and foodcoop-adam#3.

- [x] Get feedback on the concept
- [x] Clarify balancing exclamation mark for received units in balancing (see comment below)
- [x] Make update of amount of units in balancing work when units_received is set
- [x] Use ajax publish/subscribe design pattern
- [x] Find a way to minimise the risk of receive overwriting manual updates done in balancing
- [x] Edit button for the order article
- [x] Hide/disable article amount fields in edit article for receive
- [x] Warning sign when unlock button was pressed
- [x] Tests for when received units field is disabled or not
- [x] Integration test for ordering and changing received units
- [x] Review right delta (+1/-1/v) mark
- [x] Finish i18n
- [x] update German translation
- [x] ~~workaround plataformatec/simple_form#974 or~~ revert pluralisation of activerecord attributes
- [x] only show package icon when unit_quantity>1 (edit box and delta)
- [x] make delta work when unit_quantity was changed
- [x] fix notice after receive (always shows "No new articles to receive")

**Former tasks which where reorganized or deleted:**
- [x] ~~Vertical align of cells~~ -> #247
- [x] ~~Redistribute when unlock was pressed, even if amount was unchanged (?)~~

After this, next steps would be
- allow to select a different product for an ordered article (and move the ordergroup orders with it); possibly also support a replacement in a different order for a different supplier
- add a similar screen to enter products on the invoice (perhaps somewhere in the "new invoice" screen),
- add a similar screen when closing the order, so that adjustments can be made before sending it to the supplier, with foodsoft knowing about them

![receive](https://f.cloud.github.com/assets/503804/1778972/60d108f8-683e-11e3-8697-84012371ef57.png)