Support Approximate Balance Assertions

```
commodity STK
    format 1,000.000 STK

2014/01/01 * Statement Balance
    Brokerage                       123.456789 STK @@ 100.00 USD
    Checking                        -100.00 USD

2014/01/01 * Statement Balance
    Brokerage                       0 =~ 123.457 STK
```