fix the Windows 8 implementation of the getFile method

The storageFolder.getFileAsync(name) does not accept relative paths, only a simple file name...
see http://msdn.microsoft.com/en-us/library/windows/apps/windows.storage.storagefolder.getfileasync?cs-save-lang=1&cs-lang=javascript#code-snippet-1

I didn't check, but other methods might be affected similarly...
(accepting a relative path, but using methods only accepting a name)