Ruby > Gigasecond: Iteration #1

At first I was trying to figure out how many days were in the 1 million
seconds since 'Date' only allows you to increment by day, month and,
year but after reading the ruby-doc about 'Time' I found that you can
increment by the second. I only needed to convert it back to a 'Date'
before returning.

I also added the 'GIGASECOND' constant since that value won't ever
change. Ruby also gives us a nice way to make numbers more readable by
adding underscores as delimiters.

Amazingly, my first Gs anniversary was just a couple weeks ago on July
7th 2014!