Add roundup as new tracker

Related to bountysource/frontend/issues/927

Roundup is used as a issue tracker for several projects. Some examples are:

- Python Issue tracker:  http://bugs.python.org/
- Jython Issue tracker: http://bugs.jython.org/
- Tryton Issue tracker: https://bugs.tryton.org/

It can be accessed through xml-rpc as described on: http://roundup.sourceforge.net/docs/xmlrpc.html

Feel free to ask anything required to implement the API