Compatibility across ES versions

#68 introduced version-specific behavior to a few methods. Since this might happen more often, we should think of a clean way to do version-specific stuff that avoids version checks everywhere. /cc @TwP @jnunemaker

Start by reading issue #68 and identifying the methods where version-specific behavior was introduced. Trace how those methods currently handle ES versions and document the compatibility approach needed to avoid version checks throughout the code. Done means a decided, maintainable design for future version-specific behavior.

索引モデルが issue の本文から書いたものです。