[i18n] Add currencies and translate them in English/French

[tferreira]

Requires issue #60 to be done and closed.
We may be able to display the currency depending on the current browser language with a feature of react-intl

Currently we have no clue which currency are the accounts.

• We may want to be able to choose a currency for each account (or set it automatically by react-intl detection)
• We should display the currency on the left of all values €12.10 or $324.21
• The user may want to let the currency field blank... so we need to let him choose this option and then we won't display anything.

We'll probably have to create a new column in the account table to store the currency (if it's not automatically defined). This means generating a migration file.