Issue #324: When matching against a single lot, infer the currency (blais/beancount)

[Martin Blais]

New issue 324: When matching against a single lot, infer the currency
https://bitbucket.org/blais/beancount/issues/324/when-matching-against-a-single-lot-infer

Martin Blais:

This ought to work, there's a single posting in the ante-inventory:

```
#!python

option "booking_method" "LIFO"

;; fees
2017-01-01 open Expenses:Fees:Trading
2017-01-01 open Expenses:Fees:BTCMining

;; wallets
2017-01-01 open Assets:BTC:Jaxx
2017-01-01 open Assets:BTC:Coinbase

;; bank account
2017-01-01 open Assets:Checking

2017-06-13 * "Coinbase" "Test buying bitcoin"
Expenses:Fees:Trading 1.99 USD
Assets:BTC:Coinbase 0.01878653 BTC {2661.48 USD}
Assets:Checking -51.99 USD


;; transfer from Assets:BTC:Coinbase using LIFO booking method (I want to avoid
;; having to specify the cost basis to match positions here since this is not important).
;; I want to keep the cost basis with the BTC when they are transferred to other Assets
;; accounts, since I still will have to compute P&L with them. I think losing the cost
;; basis when spending to expenses is okay (?) unless I need to book them under losses?

;; These examples consider that I know the amounts of all three postings

2017-06-14 * "Transfer" "Transfer funds to Jaxx wallet"
Expenses:Fees:BTCMining 0.00105853 BTC
Assets:BTC:Coinbase -0.00144853 BTC {}
Assets:BTC:Jaxx 0.00039 BTC

```

Responsible: blais