Allow cask instance methods within blocks

Per #2449, a Cask author cannot invoke `destination_path` within an `after_install` block.  It seems logical that if the `after_install` block is deferred until runtime, it should default to accessing instance methods, as this patch provides.

Ultimately, I'd like to add more DSL shortcuts into `after_install` as is proposed for `caveats` in #2426.

This patch covers `after_install` and `after_uninstall`.  The `caveats` stanza is treated separately.