Bitcoin currency support

[diogocp]

Please add support for the Bitcoin currency. It does not have an official ISO 4217 currency code yet, but XBT is commonly used.

[chrisberkhout]

For inspiration: there's a patch I use to do it for GnuCash desktop.
It doesn't add the maximum precision allowed by Bitcoin, but works well enough in practice.

[rivaldi8]

@chrisberkhout Thanks for the patch! However, simply adding the currency won't be enough. There's still the problem reported in #635, to be solved by finishing the migration from Currency to Commodity.

[rivaldi8]

Fixed in develop (947cd7d).