Provide ImmutableDict & ImmutableMultiDict factory methods which ban duplicate additions

[qpwo]

Branching off #49, in what manner should duplicates be banned from dicts and multidicts?

Some potential options:

• Error only when key1 == key2 and value1 == value2 (I think this one)
• Error only when key1 == key2 but value1 != value2
• Error when key1 == key2 regardless of values

I'm assuming we want both a new constructor and a new kwarg for each data type. Something like constructor immutabledict_from_unique_BLANK() and kwarg forbid_duplicate_BLANK where BLANK is either keys or items.

[gabbard]

From past experience, there are situations when you would want to raise an exception in either of the last two cases; I have not encountered a situation where the first option is useful.