ENH: Implementation of compute_mean_img

fixes #182
BUG: old numpy versions
DOC: image.mean_img in examples

This actually removes a lot of code.
DOC: add docstrings
TEST: test with files too
DOC: more precise docstring
DOC: add pointers to the function
ENH: resample only mean img
Cosmit
ENH: don't cache fast operations

Address @AlexandreAbraham's comment
TST: test resampling in mean_img

As suggested by @eickenberg
BUG: address suboptimal mean_img
Cosmit for PEP8
TEST: capture warning

As noted by @VirgileFritsch
BUG: resampling in mean_img
Merge pull request #185 from GaelVaroquaux/mean_img

MRG: Implement a function to compute mean img