Added if_generation and if_metageneration parameters to set_contents_from_*
methods on gs.key.Key class.
Refactored common test functionality into a base class and added retry logic to
test cleanup.
Fixed gs.bucket docs and added [if_][meta]generation to set_acl and friends.
Minor fixes from mfschwartz.
Merge pull request #1251 from jterrace/gs-key-conditionals-rebase

Added generation and metageneration conditionals to set_* key functions.