Test return code for uploading files

[pdurbin]

I'm seeing if resp.status_code != [code] at https://github.com/IQSS/dataverse-client-python/blob/f84b980545aec73325eba88417c36a61deb79a9a/dataverse/dataverse.py#L111 for example but we should probably test the return code for adding files, especially since @chrisseto mentioned in chat today that "Our code currently expects that endpoint to always return a 201 but we just started to see 200s" http://irclog.iq.harvard.edu/dataverse/2015-09-16#i_24090 . The Dataverse installation in question was https://dataverse.harvard.edu running v. 4.1 build 71 (deployed a few months ago per https://github.com/IQSS/dataverse/milestones?state=closed ).