Push tags right after we create them.

This is tracked as Issue #24. See also
6c2947bba2af5cbc82322e0723d9bcdf11ed8357 for an alternate
implementation.
Added a post-tag hook.
Added post-finish hook.
Merge branch 'github/issue_24/push_tags'. Resolves Issue #24

Merge Yves's commit for pushing tags right after they're created. See
https://github.com/git-deploy/git-deploy/issues/24
Make the make_dated_tag function private

This function hasn't been used by git-deploy since f348aa2, add a _
prefix to it to indicate we only use it in Git::Deploy itself.
git-deploy: fix compilation error in f348aa2

The f348aa2 commit didn't compile because we weren't passing
$remote_site around.

Maybe we should just move the do_interactive_revert function around to
avoid this:

    bin/git-deploy:161:sub do_interactive_revert {
    bin/git-deploy:768:                do_interactive_revert( $remote_site, $list_count, $prefix, $action, $ignore_older_than, $include_branches,