Index __sub__ should check for instance of Index

[jasonzutty]

add currently behaves like this:
def add(self, other):
if isinstance(other, Index):
return self.union(other)
else:
return Index(np.array(self) + other)

but subtraction falls straight through to diff.
so if index_object is an Int64Index, the following behavior takes place:
index_object + 1 => add one to all indices
index_object - 1 => error in diff
index_object + -1 => subtract one from all indices

[jreback]

guess that is a bit inconsistent. Would appreciate a pull-request.

[jreback]

going to be moot after #8226

[dsm054]

Looks like the #8226 changes indeed rendered this moot.

[jreback]

PR for validation of this would be great :)