Fix several UXSS issues

[JordanMilne]

This patchset fixes the following issues:

• https://github.com/gildas-lormeau/JSONView-for-Chrome/pull/49
• http://blog.knownsec.com/2016/04/%E6%98%8E%E6%9E%AA%E6%98%93%E8%BA%B2%E6%9A%97%E7%AE%AD%E9%9A%BE%E9%98%B2-jsonview-0day/
• A UXSS via eval() in JSONView's content script which was reported to Gildas / Google privately.

[alecgorge]

I can confirm this works. I like this extension a lot. No other JSON viewer puts the path to the current item. Thanks for your work @JordanMilne !