Use os.path.join instead of urllib.parse.urljoin, fixes #12
Only use os.path.join for local files
Merge pull request #13 from jhamrick/os-path-join

Use os.path.join instead of urllib.parse.urljoin, fixes #12
Add Jessica B. Hamrick as a CONTRIBUTOR
Version 0.3.1

Better support to local data packages. Local paths do not behave
in the same way as urls (trailing backslash is different). This
version makes datapackage work in a more common sense way for
local data packages.