Add very primitive algorithms

* Sjekksum::Primitive uses modulo 9
* Sjekksum::Primitive uses modulo 97 and recudes with modulo 9
Add information about this gem
Allow string input of integer digits

Keep in mind: All non-digits are thrown away!
Refactor number converter
Update readme for string inputs
Add UPC's check digit algorithm

NOTES

    It's like Luhn, but uses simpler multiplication by 3.
    It also must allow stringified numbers for leading zeros.
Bump version