Rename Repromise.then_ to Repromise.andThen

[aantron]

See discussion starting from #22 (comment).

This is pretty much a search-and-replace task through src/, test/, and doc/. The only thing to watch out for is that some instances of the text "then_" are actually referring toJs.Promise.then_, and those shouldn't be replaced. They are always prefixed with Js.Promise, though, so should be easy to see.

This is a nice easy issue for taking a glance at the whole codebase.