documentation for `get_expiry()` and `check_expiry()`
print warning if whitelist expires soon
Merge pull request #387 from reneme/feature-expiry_warning_in_transaction

Whitelist Expiry Warning in Transaction

When entering a transaction with a whitelist that is valid for less than 12 hours, cvmfs_server will print a warning but will still succeed. Since there is a race condition in the following workflow:
cvmfs_server transaction => [whitelist expires] => cvmfs_server publish
this might come in handy when dissecting log files.