v0.2.0

Added

• A ServiceExitCode::NO_ERROR constant for easy access to the success value.
• Add Service::start for starting services programmatically.
• Add Service::query_config for getting the config of the service.
• Add ServiceInfo::dependencies for specifying service dependencies.

Changed

• Changed service_control_handler::register to accept an FnMut rather than just an Fn for the
  event_handler closure.
• Upgrade to Rust 2018. This raises the minimum required Rust version to 1.31.0.
• Replace error-chain error library with err-derive. So all error types are changed.
• Change ServiceType implementation to use the bitflags! macro.

Fixed

• Fix invalid pointer manipulations in service creation routine in ServiceManager.
• Fix memory leak in service_control_handler::register that did not release event_handler in
  the case of an error.
• Treat FFI return code 0 as error, instead of treating 1 as success.