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

Previously "classes" was hardcoded as a lib in the eclipse classpath template. 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.  However if for whatever reason, leiningen has compiled classes to the :compile-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.