Remove writers from Account and Account::Instance
Obtain the currency from the account
Store configured account definitions in the Account class
Remove DoubleEntry::currency method
Remove 1st param from Account::account method
Store configured transfer definitions on the Transfer class
Remove 1st param from Transfer::transfer method
Don't need to specify the full namespace
Merge pull request #72 from envato/currency_interface

Remove `DoubleEntry::currency`