MRG: mmap keyword for loading images

Add mmap keyword to control memory mapping on image load.

API as discussed on the nipy list.
Yeah - I removed 'r+' from the image memmap options because the image files get opened (by the image code) using 'rb' - and in that case attempting to mmap with mode='r+' will throw an error, which is rather confusing.  So routines outside the image classes can use 'r+', but not the image class methods.