Changed the MultiValueEnum's implementation to a much simpler one,

based on Ethan Furman's suggestion. (http://bugs.python.org/issue22339)
Behavior did not changed, except for lookups, which are probably a
little bit faster, because don't have to iterate over all the values,
because every single value is cached in the _value2member_map_ dict.
Also rewrote CaseInsensitiveMultivalueEnum.
PEP8, realign comment
Added one more test to maximize coverage.