tests: key of type integer

Signed-off-by: Jiri Kuncar <jiri.kuncar@gmail.com>
dictdiffer: support for int keys

* Improves `dot_lookup` function to support both dotted string lookup
  or list with keys.  (closes #12)

Signed-off-by: Jiri Kuncar <jiri.kuncar@gmail.com>
tests: check basestring handling

* Checks issue when basestring was handled as iterable. (addresses #6)

Signed-off-by: Jiri Kuncar <jiri.kuncar@gmail.com>