Use timezone aware datetimes in fixture
Declare Oscar as an optional dependency

Further reading:
http://pythonhosted.org/setuptools/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies
Update README with better links
Make gateway importable without django-oscar installed.
Use content-type text/namevalue for posts to PayPal

- Ensure payload is utf8-encoded.
- Add optional billing info payment params.
Ensure Payflow works correctly in sandbox site

A few Oscar 0.6 changes had been previously missed.
Improve dashboard txn detail template
Adjust tests to use datetimes not strings for expiry dates