build.gant: use ${eclim.plugins} for `ln -s` commands

I have setup eclim.plugins to use the dropins/ directory instead, and
therefore the creation of the symlinks failed.
create relative symlinks when possible

refs #196
clean up unix deploy a bit

- remove windows bat,cmd,exe files from the eclim bin dir
- remove *.sed exclude to chmod since eclim no longer uses sed files
- suppress error output when attempting to remove symlinks when they
  don't exist.