Refactor and merge with RESTClient cURL code

Done my first round of refactoring and code merging. I created a separate branch to do so. (7.x-restclient-merge).

A quick rundown of the changes:

- Broke up the request method into major parts (i.e. set headers, set options, set method) to more easily test and read the code
- Added new array parameter to accept any curl options
- Added admin UI toggle to override drupal_http_request (requires Drupal 7.21 or higher)
- Switched from using array of settings to simply calling curl_setopt
- Added request debug mode. Uses dpm() to output the curl http request/response object when enabled. This can be toggled on/off on the admin page

More to come.

Next up is to optimize the header parsing. Regex are expensive and we can get away with using substr which is much faster. We simply use the header and content length values coming back from the request.

Done.

Now to backport to 6.x and see about making an 8.x version.