support REST authentication on the client

[flavioribeiro]

some extra headers needed to be passed if the api endpoint is not entirely open:

Authentication and REST
When authentication is enabled on the Elemental Live system, additional information must be sent with the REST command in order to properly authenticate the request. The following additional headers must be set: X-Auth-User, X-Auth-Expires, X-Auth-Key.

The X-Auth-User header contains the login of the user to authenticate.

The X-Auth-Expires header contains the Unix timestamp (in UTC) that indicates the time after which the server will no longer accept the request as valid. For security purposes, Elemental recommends that this value should be ~30 seconds in the future.

The X-Auth-Key header should be constructed using the following algorithm:

md5(api_key + md5(url + X-Auth-User + api_key + X-Auth-Expires))