Added image upload binding (update_with_media api method)

Splited _pack_image method (in api.py) into two parts:
* _pack_file reads file and compiles POST body and headers
* _pack_image runs logic related to user's avatars and background images, calls _pack_file inside

Updated bind_api method (in bind.py) to use 'upload.twitter.com' host when 'with_media' arg was passed

Added update_with_media binding, that uses _pack_file to build postdata and passes 'with_media' to bind_api to switch host