Implement non-default bases, openexchangerates.org integration, admin improvements, and more

[jleclanche]

• Added Currency.is_base field. This allows separation of base and default currency.
• Integrated optional openexchangerates.org support through manage.py commands initcurrencies and updatecurrencies
• Increased Currency.name and Currency.symbol max lengths up to 35 and 4 chars, respectively
• Removed the already-deprecated "currency" template variable from the context processor.
• Removed the EUR fixture as initcurrencies takes care of that
• Several admin improvements

This constitutes proposed version 0.3.0.