a time_length method

Following https://github.com/hadley/lubridate/pull/283, a generic `timespan_length` method working with period, duration, difftime and interval objects.

I was not sure about the name between `timespan_length` `time_length` or `span_length`. The later was not intuitive for me.

I didn't change `int_length` yet. Do you want to keep it for backward compatibility (in that case it could be just a wrapper to `timespan_length`) or do you want to remove it?

Note: currently a S3 method. Should it be changed to a S4 method?