nth uses islice on keyerror
nth only indexes if input isinstance list, tuple
nth uses getslice to test indexability

Recommended by @eriknw in #62
nth depends on isinstance rather than __getslice__

testing for a __getslice__ attribute is faster but is not supported
by Python 3

Testing for isinstance(seq, collections.Sequence) is accurate but slow
(about 5 times slower than the current solution)

Testing for isinstance(seq, (tuple, list, collections.Sequence))
is accurate and fast in the common case.
Merge pull request #117 from mrocklin/nth-dict

Nth dict