Various changes for Redmine 1.x and 2.x compatibility

Recently some users of an internal redmine offer wanted to try out your plugin, which seems great at first place but didn't turn out to work well on recent Redmine versions. I updated the plugin yesterday and this morning, let me know if you want to merge things back and if you're interested in further update I'll make (I'll probably enhance the plugin and make it Redmine >2.0 compatible only in the next few months).

Note that I changed one functionality : I removed the permission you introduced to decide if a role should appear in the list, as I didn't understand really well the point here.

Btw, thanks for the original implementation and the idea, it seems my users are quite happy with this improvement !