Feature request: configurable currency

[martijnv]

The currency that is displayed is now dependant on the locale settings. I've managed to show EUR in all languages by changing the locale yml files. However, I ideally I don't think the currency for contracts should depend on the language that you are using Redmine in. It might be a good feature if you can set the currency for each contract, especially if you have business in various countries with different currencies and you have users that use Redmine in their native language.