applies_to_tax_exclusive_prices has been removed in f03a7e8

Cherry-picked from #1414.
Fix typo in offers docstring

Cherry-picked from #1414.
Allow access to all non-voucher offers in offers dashboard

Cherry-picked from #1414.
Don't create objects in the database unless necessary

Conflicts:
	oscar/apps/offer/models.py
	tests/integration/offer/combination_tests.py
	tests/integration/offer/percentage_benefit_tests.py

@maikhoepfel duplicated the proxy class short-circuit logic for
Conditions as part of this commit.

Cherry-picked from #1414.
Apply offers in order of descending priority

This already worked only within site offers by way of default model
ordering. Site offers are probably the most common offers, however the
ordering of all offers has to be controlled by the applicator.

Conflicts:
	tests/integration/offer/applicator_tests.py

Cherry-picked from #1414.
Speed up Travis test runs via multiprocessing

This change lets Travis take advantage of the multiprocessing plugin,
running it on all cores the Travis VM can offer us.