respect project.clj's :compile-path in eclipse .classpath

Previously "classes" was hardcoded as a lib in the eclipse classpath table. This was a bit annoying as lein's default compile path is target/classes

This fix uses the project.clj's :compile-path and adds it as a "lib".  In keeping with the previous logic, if the folder is not there it is not added to the class path. (There is no entry, so no need to remove "classes" from Eclipse's build-path) However if for whatever reason, leiningen has compiled classes to the :target-path before idefiles is run, this folder will be added as a lib on the classpath.

I am not an idea user, but I looked for the corresponding functionality in the idea config files and didn't find anything.