Add sinceEpoch and asSeconds

[Kleidukos]

@chessai I've taken the liberty of filling in the next release's changelog. Please tell me if the numbers are not correct.

[andrewthad]

I think that sinceEpoch makes sense and is a good addition. However, asSeconds creates a Timespan with the wrong implied unit. The unit for a Timespan is always supposed to be nanoseconds. If it isn't, then when you add a Timespan to a Time (via the Torsor Time Timespan instance), the result is meaningless. It would make more sense to have:

asSeconds :: Timespan -> Int64

[Kleidukos]

@andrewthad Perfect, thank you!