Ignore classpath elements when it's neither JS, JAR or directory (probably file does not exists).

If I simply enable the sbt-web plugin in the scalajs-tutorial project, I get the following error:
```
java.lang.RuntimeException: /devel/scalajs-tutorial/target/web/classes/main (in classpath) is neither JS, JAR or directory
```

sbt-web adds `target/web/classes/main` to the sbt classpath; but does not creates that folder if it's not needed; and so there is an element in the classpath that's neither a js file, a jar or a directory because it simply does not exists.

I think the scalajs compiler should just ignore that kind of classpath element instead of reporting errors.

To solve the problem, I modified readEntriesInClasspathElement to return an Option, and made traverseClasspathElements igonre the 'None' case.


We should introduce a dummy version here (should be provided by the `FileSystem` to be sure it never collides with a version of an existing file). Otherwise an appearance of an element in the classpath together with one disappearing might give the same version --> bad.