Ability to redirect donations to upstream

[thiloplanz]

An option to automatically redirect part of (or all) of your incoming donations to other people (presumably projects upstream to your work).

• need some mechanism to prevent or resolve cycles
• only works if upstream is also claimed, if not, you can still register it, but it won't be active

The ability to redirect 100% could also convince people who do not want to accept donations to use this instead of opting out altogether. In fact, even if we don't implement this, there should be an option for opt-out folks to make recommendations to where to donate instead (on or off gittip).

[chadwhitacre]

Subsumed by #449.