[backend] Fix failing update_positions spec.

The update_positions spec failed due to it expected initial position to
be nil when created but its actually 1.
Allow stock to be transferred between stock locations
API - Fix missing translation for 'invalid taxonomy id'.

Invalid taxonomy id was not translated.
Ensure item total in checkout uses correct currency

Related to #2737
Only allow editing of order and shipments if ability permits it. Also scope shipment editing js to allow other shipment forms.

Fixes #2885
Account for current_order returning 'nil' in OrdersController#update

Fixes #2750
Replace stub! with stub.

See David Chelimsky's explanation: http://www.mail-archive.com/rspec-users@rubyforge.org/msg13270.html

Fixes #2898
Fix issue where unchecking a stock location could cause it to reuest /admin/stock_items/{id}

Fixes #2896
Amend CONTRIBUTING:

* Mention about ticket references in commit message
* Add extra rules for Ruby 1.9 hash syntax and lambda syntax

[ci skip]
Clean up README [ci skip]
Update README to point to WIKI page for Core Team [ci skip]
Improve transferring stock between two stock locations
- Refactor controller method and move the work to the stock location
  model
- Don't allow a transfer to occur if the source location doesn't have
  enough inventory
only show eligible adjustments on the backend
Add spec coverage to shipping a completed order
fix up default stock location
update adjustment states for migrating stores
add shipping_rates to existing shipments
use proper up/down to restore the shipping_method_id on shipments
Fix issue during checkout where the selected shipping rate was not being
updated
Fix stock location transfer quantity check