Allow easier local dev with canary versions.

This allows specifying the Ember and Handlebars versions in the query
string. Added to enable easier local testing against canary versions of
Ember (just run `npm start` in Ember repo) via:

/ember-performance/index.html?ember=http://localhost:5200/ember.prod.js&handlebars=http://builds.handlebarsjs.com.s3.amazonaws.com/handlebars-v2.0.0.js
Merge pull request #7 from rwjblue/allow-easier-canary-local-dev

Allow easier local dev with canary versions.