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.
* Make compress Task return paths relative to target directory.
  * 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.)
  * 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.
Too much overlap from the simple test. Can you merge these into the `simple` test?