v0.12.3

Make rxjs a dependency and document installation notes

To simplify use where a developer only wants to use redux-logic and
doesn't plan to use observables directly, making rxjs a dependency
allows redux-logic to be installed w/o manually installing rxjs first.

It is still recommended to install rxjs first if Observables will
directly be used in the project, so that multiple copies are not
installed. A note was added to the readme.