Multiple operations behave differently

Hello,

I get the following behaviour

```
dcalc now - 2016-03-09 + 14d d
25.58 days

dcalc now - 2016-03-09 - 14d d
53.58 days
```

It seems the operation works in the other way. The `+14` does a subtraction and the `-14` does an addition.

Is this the correct behaviour?
