When @wasade reviewed my ```DistanceMatrix``` pull request, he suggested that it'd be more pythonic to just check for data equality, instead of both data and type equality. I looked around on SO and in some of the python standard library code, and people appear to be on the fence about this one.

```DistanceMatrix``` doesn't check for type equality and instead defers to duck-typing. I don't feel strongly about this, but we should probably be consistent with how our ```__eq__``` methods behave when possible. @gregcaporaso @wasade what do you think?