Af convert

This adds the "convert_and_store" call to the client. It is currently lacking tests, but so far it seems to be working well. Testing this is a bit difficult since it deals with image manipulation. The tests for this should probably be addressed in https://github.com/infowrap/filepicker_client/issues/5 since we'll probably need a static image file to test against.

The new call uses Filepicker's image conversion API to generate a new file and then store it at a specified path.