Use 'exclude' in OpeningPeriodForm to allow customisation

Defining fields directly means any additional fields are not picked up.
Bump version to 0.4.1
Merge branch 'release/0.4'

Conflicts:
	setup.py
Merge branch 'master' of github.com:tangentlabs/django-oscar-stores
Use url from future

Make templates compatible with Django 1.5
Correct installation instructions in README
Allow higher versions of Oscar
Merge pull request #65