Fix jar file separator handling on Windows.

On windows, File.separator is "\", but the separator in jar file is always '/'.
Thus Robolectric can't load android res files from android-res-4.1.2_r1_rc-real.jar.

This patch fix it by add special handling for JarFsFile in DirectoryMatchingFilter#accept.

Some tests get error because of this. After this fix there is no error. (but some failures still remain.)

This patch could be related to the issue #490.