This is another go at adding proposed static multicast functions. Returning ConnectableObservable from operators is/was a mistake.

TODO:

• Settle on names (currently, multicastFrom (static), multicastWith (operator), publishFrom, etc)
• Deprecate publish operator variants that do not have a supplied selector
• Add tests
• Add more docs

NOTE:

Also adds a test showing a weakness of our current approach with regards to composing custom observable types through operator chains. This could probably get added at another time, but I wanted to be sure to capture it while I was thinking of it, and seeing some of the ConnectableObservable code reminded me of the issue.

To discuss:

• Names? xWith makes sense, but what about xFrom?
• Should we have a static variant for every permutation, or just for multicast?
• How frequently do we use these things?