Update for newer versions of Etherpad Lite

[jasonaowen]

While working on #2, I realized that I may have too tightly coupled this plugin to the 1.8.7 release of Etherpad Lite. In particular, it seems the internals of how plugins are installed changed with 1.8.8.

Make sure that the plugin works with newer versions of Etherpad Lite, and update the documentation for doing so.

See also commit ether/etherpad-lite@da10d42