expose stamp in arakoon_client package

[domsj]

fwiw, the travis failure:

==============================================================================
Error: universe:11:system:1:elect_master:1:nothing_on_slave
Failure("No Master")
------------------------------------------------------------------------------
==============================================================================
Error: universe:11:system:1:elect_master:0:all_same_master
Failure("No Master")
------------------------------------------------------------------------------
Ran: 131 tests in: 491.69 seconds.
FAILED: Cases: 131 Tried: 131 Errors: 2 Failures: 0 Skip:  0 Todo: 0 Timeouts: 0.

[toolslive]

LGTM