Make Category-parent-relation unique. Fixes #48
Adding encoding header to Python files. Fixes #46
Update Add new transaction view to new model.

In case of no on_stock value has been defined for an storage item, just no decrement is happening.
Merge pull request #49 from frlan/feature/SKU-based-storage

Add SKU and StorageItems based basic storage mangement
Add to to add a new part view
Add links for storage management to base template
Adding basic class for storage related views
Add a page to show all storage items and reorder a little inside base.html
Fix breadcrumbs for storage item list
Add a storage item detail page
Link storage item detail page to storage items list