Remove Oscar from install_requires

Instead, we assert a minimum version before installing.
Bump version to 0.7
Merge branch 'master' of github.com:tangentlabs/django-oscar-paypal
Log all PayPalError

Logging is done just before the exception is raised. There is a limited
number of times when PayPalError is rased but there is much more views
handling PayPalError.
PEP8 adjustments
enhanced paypal express params handling
Don't use mutable defaults

Plus a few other comments and tidying tweaks.
Prevent freezing the basket for Express redirects with errors
Use https for callback URL when DEBUG=True and in sandbox mode