Add ability for :libs and :externs to be pulled from both the file system and the classpath

Currently :libs and :externs pull from the file system only and
have no option to pull from the classpath for resources. This makes
it almost impossible for someone to develop a library that has
external dependencies that are not clojurescript (goog.dom.query) and
deploy it as a jar or zip. The attached patch adds support
for classpath resources resolution to make it easier to bundle these
resources in with the libraries.