Limit public objects of API modules.

This only exposes the objects we really want to be exposed to users of
the API, something like for example ValidatedHTTPSConnection or
SSHAskPassHelper shouldn't be used by projects implementing the API.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Warn about using RescueSystem.shell().

This method is going to be removed from the API in the next major
version and wasn't intended to be in the API in the first place.

Prepares for addressing issue #13.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>