Made fileno transparent
prevent memmap being created from .bz2 file
Test that unzipped nifti files return memmory map

Also tests that zipped nifti files return ndarray.
Tests now also check save/load for unzipped nifti files, besides the
zipped version
Merge pull request #258 from MichielCottaar/fix_mmap

MRG: Allow unzipped nifti files to be loaded in numpy mmap

In the latest numpy version (1.8.2) and perhaps in earlier numpy 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 accessible in the Opener object and thus causes ArrayProxy.get_unscaled to return a memory map (as originally intended) rather than an array for unzipped files.