Added support for a default file system.

FileLocatorUtils now defines a constant pointing to the default file system
instance. There is a method which checks whether a file system is set. If not,
the default file system is returned. This mechanism is going to replace the
static default file system field used by the FileSystem class.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1523796 13f79535-47bb-0310-9956-ffa450edef68