modifications to allow multipart-form-data POSTs

- multipart-form data POST requests now possible for both Oauth1 and 2
- POST request function takes extra boolean argument to indicate
whether multipart form data or not
- for three-legged oauth 1 flow, POST function also takes extra
argument to limit parameters to be signed
- three legged 2.0 oath looks for ‘display’ parameter which is useful
for Facebook