#16 Auto-copy files to be modified

[Elliotte Rusty Harold]

We should to rely on the user to make a copy of the
source directory. Jester should make its own tmeporary
copy of the sources it's going to mutate and compile
those. Furthermore it needs to add that directly to the
classpath itslef, rather than relying on the static
classpath. This is important for both safety and
usability.