Updated rspec syntax and spree namespacing for readability.

Fixes #2777
admin tabs are displayed only if user has :admin right on resource

Fixes #2626
Fixed absent money spec from tests.

Fixes #2778
Specify :class_name options for InventoryUnit associations so that assigning them doesn't fail

This is due to an AR bug which assumes the class is not namespaced.
Make ProductDuplicator#duplicate_image actually return an object, not nil

Fixes sub issue in #2684
Specify :class_name for has_many images on Variant

Previously, attempting to assign invalid data (i.e. variant.images = [nil]) to this association would've resulted in an uninitialized constant error for Image, rather than erroring correctly on an association class mismatch.
Changed line items to display cost-per-unit using the single_money method instead of display_amount from the associated variant

Fixes #2772
nested_taxons_path will now take a Taxon object and generate a proper URL
stock locations and items
syntax update for factory girl 4.2
create default stock location and migrate variant counts to stock
use factory girl inheritance for order factory
default packaging for stock_locations
add variants with status of backordered or on_hand to packages
properly calcualte on_hand and backordered stock
extract logic out of stock_location model into the PACKER
move stock packages and splitters into stock directory
move splittrs in to module subdirectory
chain of responsibility for splitters
split packages by backordered status