Add addition for words
Implement addition of two Integer types
Add a public API for addition
Add ten's complement
Add a public API for subtraction

Subtraction is addition of the ten's complement of the second number.
The definition of ten's complement for multi-word numbers is a bit
unclear in this commit and will need to be clarified.
Add increment and decrement operators
Merge pull request #77 from joichiro/addsub

Addition, subtraction, and signed arithmetic