moved string module inclusion from String to NSString.

So far, BubbleWrap::String was not applied to NSString.
This causes problems when working with methods which returns NSString like JSON#parse.

ex)
hash = BW::JSON.parse('{"color": "#002244"}')
str = hash['to_color']  # str is NSString not String
str.to_color      # raises undefined method error because str is NSString

# str.class returns String, but actually not.

now it works because BubbleWrap::String is included into NSString.