added ability to specify request options via config

this allows the consumer to supply additional options to the http.get request call by doing
```
   mixpanel.init('....', { request_options: { scheme: 'http' }});
```

The longer story is, i'm using the module from within a chrome extension.  That extension sometimes makes mixpanel calls from a window whose protocol is `chrome-extension://`.  Since no scheme is specified, `http` uses the document's scheme, so the request ends up being `chrome-extension://api.mixpanel.com...`.   fail.

Updated tests, everything still pases.

zeev