Leave body empty on custom action with no properties or data

WinRM uses custom actions and errors out if there's an empty %s_INPUT
node in the body.
Add comment: wsmc_set_options_from_uri should be wsmc_set_selectors_from_uri
Bump version to 2.4.0, libwsman_client soversion to 2.0.0
Ruby: Add accessors for ClientOptions#selectors, #properties, #options

Plus plenty of documentation improvements
Define delivers security modes (WSMAN_DELIVERY_SEC_*)
Full WinRS client implementation in Ruby