Add RequestError method to pass along request attributes

For "web apps" it would be nice to pass along information about the request to rollbar for indexing and such.

This PR adds two methods `RequestError` and `RequestErrorWithStackSkip`. They are the same as the existing similar methods, but add the request as the first parameter and populate the data to be sent to Rollbar. Populated fields includes: url, method, query_string headers, and post data.

The [Rollbar item post documentation](https://rollbar.com/docs/api/items_post/), for reference.