Add help option and range check with some whitespace cleanup

I liked the start of your nagios-plugins library for use in ruby; I was trying to write a nagios check to work w/ puppet's facter library and this seemed like a great start.

Using the --help option and range check I added (pulled from here: http://pastebin.com/RAZkxhY2 w/ some cleanups) I was able to produce this (mostly untested) plugin:

http://pastebin.com/RAZkxhY2