Using Oscar's load_class to get ProductCategoryView

The Oscar contrib views hard codes the import of ProductCategoryView.
This proves problematic when attempting to override the view in a
local project.

This change uses oscar.core.loading.get_class to obtain the view
instead.
Merge pull request #19 from a-musing-moose/refactoring

Using Oscar's load_class to get ProductCategoryView
Switch to released version of Oscar
Update requirements for testing
Add requires.io badge for outdated requirements
Split requirements for testing and general development into separate files
Restrict django-model-utils version with Django 1.5.5 tests