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.
Merge branch 'master' into cachemixin
Merge branch 'master' into cachemixin
Renamed CacheMixin.cache to _cache

Corrected a bug in _cache() as well: return the input function
unchanged if self.memory is None.
Documentation fix
CacheMixin._cache always returns a Memory object.

All default 'memory' parameters have been set to Memory(cachedir=None)
for _cache() to always return a Memory() object.
Merge pull request #41 from pgervais/cachemixin

Cachemixin
Merge remote branch 'nisl/master' into api_cleanup

Conflicts:
	nisl/utils.py