Hook parts into new Spree stock API

So that they are included on shipments and have their inventory units
changed as expected. Most of the previous code was completely removed
since it didn't make sense anymore
Allow both Spree 2.1 and 2.0 to use master branch

No need to create another branch now if we can keep compatibility with
both Spree versions on the same branch
Improve test coverage with features spec
Display included parts on completed order page
Update README
Fix backend shipments UI

Use deface to override parts of the shipment partial as it considered
every item in the manifest to be an order line item. But that's might
not be the case once the store starts building bundled products

Fixes #31
Use an icon for the Parts tab
Add failing spec for orders in backend:

This adds a failing spec for orders with both:
- A product assembly
- An item that is a product that also belongs to that assembly
Viewing such orders in the backend causes an sql exception due
to the ambiguos semantics of the assemblies_for method in such
contexts.
Fix variant.assemblies_for query
Make it use all Spree components.