Changes to Default Currency

In settings:
Instead of typing in a currency code, the admin can now select a currency from the list. I left the underlying value unchanged as the cur_code (though I think perhaps the id would make more sense), so this shouldn't have any ramifications elsewhere. I also set that list to be updated whenever a change is made in currencies.

Everywhere else:
Previously, everything defaulted to a hard-coded currency id 1, regardless of the Default Currency Code in the admin. Wherever I could find it, I removed the 1 or changed it to NULL. I then let the currency_m model grab the default currency if none is specified. I removed all the 1s that I found, but it's possible I missed a few.