Provide configuration option to not try memmap'ing in array_from_file by default

[yarikoptic]

As suggested in http://lists.alioth.debian.org/pipermail/pkg-exppsy-pymvpa/2014q2/002790.html it seems that memmapp'ed access to a .nii file might be slower (I will seek more details about system in a sec). Altogether it might be worthwhile to allow for explicit control either volume should be memmapped or loaded into memory (I guess a kwarg memmap in (None, True, False) to be carried all the way into array_from_file)

[matthew-brett]

Yes, I agree, we need that. I think it should be a kwarg to 'load' then passed into <image_class>.from_filename, thence <image_class>.from_file_map, and finally into the proxy creation object. Sadly would probably therefore need tests in test_image_api.py and test_proxy_api.py...

[matthew-brett]

Implemented in #268 - would you mind having a look?

[matthew-brett]

Closed by #268