Feature: Display prices in user's prefered currency using conversion rates

[SweetMNM]

I personally have subscriptions in 3 different currencies, and especially in the reports page when trying to figure out monthly and yearly costs it makes things inconvenient.

2 enhancements to take this even further that will be nice but not as important as the main feature:

• use conversion rate of past dates meaning instead of just converting everything with the current rate convert each billing for the bill date
• assuming Add ability to add payment method to the subscriptions #70 gets implemented it would be nice to assign a conversion fee for each payment method, which would get us closer to displaying accurate prices