Implement 'assert_exists' support

See: ARAKOON-342
See: http://jira.incubaid.com/browse/ARAKOON-342
Closes #6
See: https://github.com/Incubaid/crakoon/pull/6

Change-Id: Ia893f515bda256c56f4ada5742fc4e4b2902db84
Signed-off-by: Nicolas Trangez <ikke@nicolast.be>