Little css tweaks
Better highlight of active file in diff tab
Make PR tabs scrollable if too much un line

Fixed #197
Conmits tabs can be closed

cf #105
Replace memoize (process) by our cached_method decorator (instance)
Manage commits from the future
Tabs are present on issues if a commit is displayed

cf #105
Make click on commit comments count open commit in tab

cf #105
Make prev/next comment work on diff with on one file

(done by always show the files list, even if only one files)

cf #105, but limited to commits (but there is more chances to have a
commit with one file, than for a PR)