Kernel#system calls avoid shell

cp(1) invocations are replaced by FileUtils.cp, remaining
Kernel#system calls are done with a list of arguments instead
of a single string.  this way the code ends up using fork/exec
instead.

security, correctness FTW.
log output for mogrify less confusing

it used to output the full command line, but that can be
pretty confusing if any parts of the command need quoting.

previous implementation simply broke in such a situation.

current output is similar to the one for page transformations.
Merge pull request #10 from roman-neuhauser/system-no-shell

Kernel#system calls avoid shell