Changed base.__pow__() to use O(lg N) binary power algorithm.
Fixed bug in new spmatrix.__pow__
Added a test for __iter__(). Need to decorate for types that don't support that method.
Overrode test_iterator() for DIA and COO since they don't have __getitem__
and added @dec.knownfailureif for BSR
Merge pull request #513 from Daniel-B-Smith/master

ENH: sparse: small performance improvement in __pow__

Use O(log N) rather than O(N) algorithm