Add a builder API for Composable

[jbrisbin]

Due to the recent constructor injection changes, it's not very easy to create a Composable that is based on an existing Reactor or that uses a specific Dispatcher implementation.

It would be nice to have a general-purpose builder API for Reactors and things that need Dispatchers so a Builder class that's independent is probably the best long-term way to go.

For the time being, though, I suggest we add a Builder inner class to Composable and have the static from method return this Builder just to provide a quick and easy alternative to what was previously possible by using the from method, then calling setDispatcher.