Fix calculations for large numbers. Fixes quicksilver/quicksilver#1572

Previously we were only showing 4 significant figures. I've updated this to 20.
This meant that things like 10! were truncated to 4s.f. which is silly.

Other changes:

* Only show a reasonable number of decimal places
* Format the answer to be how the user inputted it (with the right decimal separator and with grouping) (i.e. if the user inputted `100,000/3` the answer would be `33,333.3` - depending on the user local's decimal and grouping separators)
* Show a notif if the calculation fails