Webapp startWebRequest

![screen shot 2014-12-02 at 1 39 39 pm](https://cloud.githubusercontent.com/assets/5429146/5271395/d17e8862-7a28-11e4-87a0-d0b5932aac7f.png)

* added new startWebRequest API which calls GET on a URL. Asynchronously, the caller is called back with the status code, content type, and content of the response.
* exposed JSON.parse() in the sandbox so it can be called to parse a JSON response.
* fix some styles on IE
* replaced a call to Blockly.fireEvent with the equivalent code that doesn't require blockly-core