PerStateOutput plugin

So here is a simple plugin that I find useful during my experiments with S2E: splitting debug and log output across one directory for each symbolic state instead of having everything appended into one monolithic debug/log/etc. file.

I'm sure there are much more elegant ways to implement this; I'm all ears for suggestions.  I can, however, confirm that this worked satisfactorily for me...

Signed-off-by: Diego Biurrun <diego@biurrun.de>