SumOfNaturalNumbers: Simplify the triggering.
SumOfNaturalNumbers: Faster computation.

- Don't use List::Util for computing the number.
- Stricter regexp.
- Add attribution.
SumOfNaturalNumbers: Add a "from" in the regexp.

So that it can accept "add from 1 to 100"
SumOfNaturalNumbers: Change colon to "is".
Sum: Add a period at the end.
Merge pull request #368 from duckduckgo/jag/sum

Sum of Natural Numbers: Review for Release