Require Spree 2.1.0.beta
Remove references to methods removed in spree/spree

So that we can actually boot the app
Spree::Variant no longer has a `on_hand` attribute

Instead stock it should be check through variant stock items. This makes
it possible to go through checkout without raising errors
Allow test to run

Update factory girls and test helpers path
Remove specs that give no value to the extension

They were basically testing spree core functionalty. No need to maintain
those here
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
- wip
- WIP: variants can now have parts
- WIP: variant parts