v0.4 release candidate

This is a release candidate for v0.4. This version of xray includes some major changes, so I wanted it to get some testing before its official release.

For a list of changes, please read the release notes from the development version of the documentation:
http://xray.readthedocs.org/en/latest/whats-new.html

To test it out, use:

pip install https://github.com/xray/xray/archive/v0.4rc1.zip