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

yes, it seems like a good practice. In this case, these are the opts from the original command. All we need in the callback (currently) is the passed-in callback function (func), but some of the other scenarios require other parts of the command. I'll start adding some comments about this. Can you point me to an example of how you like to do it?