FileSystemUtils#freeSpace can be simplified by using new Java 6 File methods

Commons IO now requires Java 1.6, which has some useful new File methods:

File.getFreeSpace() / getUsableSpace()

These could be used to radically simplify (and speed up) the FileSystemUtils freeSpace methods, as well as making it OS-independent.

The methods should be deprecated in favour of directly calling the File methods directly. Perhaps even deprecate the entire class? - it only contains the freeSpace methods currently.

Note: a lot of the tests will becorme redundant or need reworking.