Use filename variables in examples

to make it more obvious that the *_img functions can take filenames as input. In general it is not very clear what kind of object bunch.cmaps[0] is.

Other minor fixes went in while I was at it, amongs these:

- use dataset consistently for the object return by fetch_*
- use from nilearn.datasets import fetch_... to avoid dataset (the object returned by the fetcher) being confused with datasets (the module)
- pep8 fixes here and there

Comments more than welcome!