Especify ruby 1.9.3 in gemfile and use require_relatives as needed.

It wasn't clear what version or ruby to use(it seams it was 1.8.x),
so it was really hard to even test it and/or make any changes.
Also were add require_relatives to be able to test it without having
to install the whole gem everytime(maybe due to the ruby version).
Fix bug with invalid characters in parse_number and add the proper test.

ItuCodes.parse_number were returning non-numeric characters, so now
it is calling clear() in the input number before procesing it.
It was also add new test to check this.
Added method north_american_area_code_for and add the proper test.

This method returns the area code for a given north american cellphone
or nil if the cellphone is not north american.