Add listenToOnce

[willdady]

I think it would be a good addition for stores and actions to support a listenToOnce method which automatically removes it's callback once triggered. See backbone's events for example.

[LongLiveCHIEF]

+1. This will also be something that will potentially be mitigated within the POC concept we're working through for the 0.3 release, but if not, I will definitely add it in there. Great idea!