remove history handling from `cvmfs_swissknife sign`
add scaffold for `cvmfs_server tag`
add `cvmfs_server tag` to usage string
allow for both read and export of unsigned manifest
check for spaces in tag names
add `cvmfs_server tag -i` (inspect tag)
Refactor: extract method `sign_manifest`
rename description parameter in tag
add setters to `Manifest`
implement `cvmfs_server tag -a` and `-r`
remove `swissknife::CommandTag`
rename `CommandTag_` to `CommandTag`
allow closure in `Observable::RegisterListener()`
TEST: add closure usage to t_util_concurrency.cc
use closured Future to retrieve content hash
add `CommandRollbackTag`
use `cvmfs_swissknife tag_rollback` in `cvmfs_server`
remove legacy `CommandRollback()`
code space cosmetics
re-introduce magic undo tags