Forbid zero value baskets

Forbid transactions with zero value baskets. See issue 22:
https://github.com/tangentlabs/django-oscar-paypal/issues/22
Correct template variable references
Bump version to 0.6
Add i18n to docs example
Improve more fields in dashboard templates
Add find_by_code method to shipping repository

This is needed to ensure the shipping method is looked up correctly when
using PayPal Express as a payment method only.
Look up shipping method name from session (when appropriate)

It appears to be impossible to pass the name of the shipping method to
PayPal Express when shipping amount is specified directly.  Hence, we
need to rely on the shipping method stored in the session to retrieve
the name.
Correct shipping method name assignment
Bump version to 0.6.1
Correct README grammar
fixed *AttributeError: 'SuccessResponseView' object has no attribute 'basket'* in SuccessResponseView.get_context_data in case of paypal exception during payment
Merge pull request #41 from marcofucci/get_context_error

Fixes self.basket not found in SuccessResponseView in case of paypal payment exceptions
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