examples: add a streaming HTTP export/import example

The example shows how to
  - export by HTTP GET
  - import by HTTP PUT
  - pipe an export into an import with error handling

Signed-off-by: David Scott <dave.scott@citrix.com>
We should be supplying something for the "originator" parameter to the login_with_password call for all our uses and examples. In this case probably something like "XenServer example Python streaming HTTP export/import".