refactor release fab tasks

this includes a change to activate releases by sha instead of
rollforward. rollback remains
ln -f doesn't seem to work. rm first instead
flakes
review comments: less magic
refactor rev-parse + more tests
remove indirection in purge
tests for prune
typo
bring docstring up to date
add tests for activating existing release
explicit rollback
Merge pull request #13 from onefinestay/activate_release

Activate release