Warn incompatible lazr.restfulclients

Occasionally changes will be make to lazr.restful will cause older versions of lazr.restfulclient to break. It would be nice if lazr.restfulclient reported its own version (via an HTTP header perhaps) so lazr.restful could generate an error or warning when an incompatible client connects.

The intent is to help users of lazr.restfulclient fail early when they attempt to use a new version of a web service that their client can not handle as opposed to failing late and in less straight-forward ways when they attempt to use the incompatible part of the web service.