Vistrails integration (#69)

This adds the VisTrails integration plugin.

It has two components:
* A method (`do_vistrails`) that fires after an experiment has been unpacked (`post_setup` signal) to create a CLTools module and a VisTrails workflow.
* The module itself can be run, it is called from the CLTools module, and it parses the info from VisTrails to call the different steps of the correct unpacker (upload, run, download)

This is built on top of the 'plugins' branch, see #76.