add counter_cache for performance improvements on number of shipped issues
Remove duplicate unship method

For some reason the way the last two PRs were merged caused git to add this method twice. We can safely remove the second one.
Merge pull request #63 from brchristian/patch-2

Remove duplicate unship method
Merge pull request #62 from mcsweeneys/shipped_issue_counter_cache

add counter_cache for performance improvements on # of shipped issues