Adding issue history tabs plugin

This plugin splits the issue history into two tabs, 'all' and
'comments'. The comments tab only displays the issues notes and all
displays everything (it defaults to comments).
I think this will make it easier to navigate the issue page, as some
issues' histories can can very large.
Changed default tab on issue history

Now tab only defaults to comments (as opposed to all) when there are
more than 25 entries in the issue history.
Merge pull request #69 from dannybutler/issue_history_tabs

Adding issue history tabs plugin