Add file exists check to copyfile_sparse

In acbb467709ce43b9c16b0ad2f5e025533fe8505c a os.path.samefile was added but no checks on if dest / src exist prior to it.
This is problematic especially when the destination file does not exist already. Now it only runs samefile when a dest already exists.
Add support for Ubuntu 13.04 (Stable release on 25th of April) and add in 12.04.2 (Mid Feb)
Clean up imports and various other smaller items

- Import OzException directly instead of referencing the full package name
- Import various os.path and oz.ozutil functions to more readable and shorter lines
- Replace multiple write() calls with a single multi line write() call