Manage unicode header keys in Pithos requests

Closes #10

Methods affected in kamaki.clients.pithos.rest_api.PithosRestClient:
account_head/post, container_head/put/post, object_head/get/copy/move
Make upload destination and optional argument

Fixes: #11

In "kamaki file upload", the destination path is now an optional
argument, set to be equal to the base name of the source path, by
default.