Deprecate #watchable? for #embeddable?

This is for [Issue #38 ](https://github.com/maxjacobson/film_snob/issues/38).  My apologies--I meant to ask if I could tackle this before doing it, but I forgot once I got started.

I implemented the deprecation via a Deprecation module.  It has a method to alias an old method name with a new method names while displaying a customizable warning.  

If you accept this pull request and want to remove the deprecated method, it will only require removing lines 20-21 in film_snob.rb (and you can then also remove the requirement/extension of the Deprecation module).

I've also changed the README, specs, and a condition in film_snob.rb (line 28/34) to reflect this deprecation.  All specs are passing and I've removed all Rubocop offenses.

Thanks!  Hope this helps!