Numeric operations with Duration Class

[Angelmmiguel]

With ISO8601::DateTime class you can do operations like these:

hour = 60*60
dt = ISO8601::DateTime.new('2014-05-28T19:53:10Z')
dt + hour

When you use this gem with Rails, it's very useful to work with a Numeric class because ActiveSupport provides us with a lot of time helpers: 1.month, 12.years...

I add Numeric operations feature to ISO8601::Duration class.

[Angelmmiguel]

I deleted TypeError clases because it was duplicated :)

[arnau]

Besides the comment above it looks good! Cheers

[Angelmmiguel]

Updated the documentation

[arnau]

:+1: