Core Time classes monkey patches

This PR is not ready yet, it's done to implement syntax described in issue #1 

I've implemented + and - for now using some metaprogramming but I'm not sure it's worth it doing it this way

Probably manually implementing + and - would be a better idea ? WDYT ?

````ruby
def +_with_working_hours(other)
  if (other.is_a?(WorkingHours::Duration)
    other + self
  else
    self.+_without_working_hours(other)
  end
end

def -_with_working_hours(other)
  if (other.is_a?(WorkingHours::Duration)
    other - self
  else
    self.-_without_working_hours(other)
  end
end
````
This file shall be in `core_ext` directly isn't it ?