Removed a "except Exception:" occurence

Cleanup in comments.
utils.cache -> utils.CacheMixin

Transformed the awkward utils.cache function into a method. Created
CacheMixin class to contain this method.

Examples still run successfully.
haxby_simple works with numpy 1.3 and sklearn 0.12

Added a workaround for missing features in either sklearn or numpy.
Added some user messages to the gen_rst extension

A few messages were added in the gen_rst extension for Sphinx.
Corrected some typos as well.