Update draft document

These changes makes it possible to update Draft documents that have previously been created with vim-draftin. Two dependencies has been added, but I don't expect this to be a big problem for users capable of installing vim-draftin itself. The first is to parse the JSON reply, and the second is to encode/escape the content for the upload.

The patchset also contains autocommands such that a write (:w) will automatically call :Draft on previously created documents (and conversely :Draft will write the file).