Included Arithmetic expansion for fix for #18

Mentioned Arithmetic expansion syntax `$(( ... ))` along with expr.

Also, edited the Wikibook on Bash Scripting to include expr (assuming they accept the changes) - see http://en.wikibooks.org/w/index.php?title=Bash_Shell_Scripting#expr_.28deprecated.29.
Merge pull request #44 from mrecachinas/patch-1

Included Arithmetic expansion for fix for #18