A few goodies in this commit:
 - New interceptor classes that expose more than what the Apache HttpClient interceptors could, such as the proxy request object.
 - getHar() now waits up to 5 seconds for active requests to complete before returning.
 - HAR files will now contain the actual text of the content if setCaptureContent(true) is called. The text will be the actual text if the content type is text/xxx, otherwise it will be base64 encoded.
 - A nice little functional test suite that includes a mini web server to test recent requests that have come in to the mailing list.
 - A concurrency bug fix related to custom headers.