Add support for Future

[bblfish]

The current implementation works with cats.effect.IO which is clean and pure. But it has the disadvantage of being a bit heavy for projects that rely on scala's Future and may just want to add an signing layer without buying into the full cats.effect stack.
See my longer comment in the discussion on custom IO Runtimes.

For expediency I used IO in commit co-operating-systems/Reactive-SoLiD#20 .

If people would like to use this library but feel this is a blocker please contact me to help me change the priority of this issue.