How to add sites to the Safari Power Saver exclusion list?
Thanks to Thecafremo's tip, the responsible file is ~/Library/Safari/PlugInOrigins.plist.  I found an AppleScript published by GitHub user crmne (Carmine Paolino), SafariPowerSaverWhitelister.scpt, that is supposed to add entries (https://gist.github.com/crmne/5855a901a0a9a6ef30a5).  You might try it, but it did not work for me in Safari 7.0.3.

Instead, after clicking on a "play music" button, I opened up Web Inspector and ran the following JavaScript on the console:

window.embedElem = document.querySelector("embed");
embedElem.style.position = "fixed";
embedElem.style.width = "100px";
embedElem.style.height = "100px";
embedElem.style.top = "500px";


There was a 100x100 black square hovering on the left of the page toward the bottom.  After clicking it, Safari added an entry to the exclusions list for play.google.com.