[Added signal library.
bringert@cs.chalmers.se**20051107180004] 
[Only set MVar on exceptions, not always after the thread finishes.
bringert@cs.chalmers.se**20051107181409] 
[killComputation does not use the MVar.
bringert@cs.chalmers.se**20051107181628] 
[Added runInterruptibly_.
bringert@cs.chalmers.se**20051107182212] 
[Simplified Signal, no need to spawn a new thread.
bringert@cs.chalmers.se**20051107194238] 
[Changed runInterruptibly to return Either.
bringert@cs.chalmers.se**20051107195307] 
[Changed indentation.
bringert@cs.chalmers.se**20051107200031] 
[Updated documentation to match the new implementation.
bringert@cs.chalmers.se**20051107201152] 
[Added signal/Makefile
bringert@cs.chalmers.se**20051107221735] 
[Restore old sigint handler after the action.
bringert@cs.chalmers.se**20051107222050] 
[Updated docs about restoring the old signal handler.
bringert@cs.chalmers.se**20051107222350]