Allow unzipped nifti files to be loaded in numpy mmap

In the latest numpy version (1.8.2) and perhaps in earlier number versions the memory map (mmap) requires access to the file number assigned to the file when it was opened. This patch makes the fileno method accesible in the Opener object and thus causes ArrayProxy.get_unscaled to return a memory map rather than an array for unzipped files as originally intended. This is also related to issue #9 and issue #18, because it allows partial opening of files through the numpy memory map utility.