Add convenience API (seconds)

As can be seen in ef6a18a54aee59a6c8327f0b32b06b23792fc21d we now have the base API provided by libdiscid, but having 1-2 generated attributes will help users to do what they want.

`Track.length` is in sectors, but users might want the playing time, so we should at least provide `Track.seconds` so the user doesn't have to know `SECTORS_PER_SECOND = 75`.
Providing a string or separate minutes and seconds would be a bit too much though.

Similarly we have `Disc.sectors`, but users might want to know the total playing time.


[rb-discid](https://github.com/phw/ruby-discid) is a good example in that regard.