Revert "Also rewrote CaseInsensitiveMultivalueEnum."

This reverts commit 34196351230b2c9e184ce34daa4dea9e5af20d7f.

Conflicts:
	tests/test_caseinsensitive_multivalue.py
Revert "Changed the MultiValueEnum's implementation to a much simpler one,"

This reverts commit 11a56670be3d02dbe31266d38dd48012639de109.
Revert to v0.6.0 because of a bug.

To enforce type checking at class creation time, it's only possible with
metaclasses, because single values and tuples with single value are passed to
__new__ the same way (as positional arguments), so it's not possible to tell
if it was a single value or a tuple originally. http://bugs.python.org/issue22339
Test to prove this.