Proposed updates to the target directory.

* Make the target directory always unchangeable.
* I cannot think of a reason why someone would need to change the target
directory.
* This is the same as sbt-uglify, sbt-digest and sbt-gzip.
* I have left the parentDir setting available, and defaulted it to
nothing.
* I think most users would want the generated files directly in the
target directory.
* I am not sure that the parentDir setting is useful, as the compressed
files appear in a different directory to the uncompressed original
files.
* Previously when I had source files in sub folder(s), they lost any
parent folders.  (The current code only returns the immediate parent
folder and no other ancestor folders.)
* Make compress Task return paths relative to target directory.
* This is the same as sbt-uglify, sbt-digest and sbt-gzip.
* I believe this follows 'the second member declares the portion of that
path which is to be considered relative' from
https://github.com/sbt/sbt-web#writing-an-asset-pipeline-task
* I had to update the sbt-test/simple so that it would run on Windows.
* It should still work on Mac/Linux, but I have not tested.
* I have added a sbt-test/parentDir to test that the parentDir setting
works.