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, but some may want all the concatenated files in a
sub-directory.
   * Previously I had to have my generated files in a sub folder, or the
target directory was the webTarget root, which makes the target/web
folder unclean.
* Make concatTask 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.