Non existing directory

Handle classpath entries that point to non-existing filing system directories

Currently, the library throws an exception, which causes big issues if one of these URLs is present. A directory in the classpath that does not exist may seem like a problem with the classpath, but it actually seems to be quite a common occurance. Servers might add an 'override' folder to drop classes into for example. Empty directories, or non existing directories, cause the actual Java class path no problems.

Issue seen in our code using `ClasspathHelper.forJavaClassPath()` with a directory that does not exist.

The patch is the minimal necessary to work around the problem
Fixes issue [148 at Sourceforge](http://code.google.com/p/reflections/issues/detail?id=148)