[FR] render progress bar while doing statistics

[bruceauyeung]

listing pull requests takes lots of time. we should consider adding progress bar while doing statistics.
considering that we have a config statBeginTime to avoid listing all pull requests, we can not design a progress bar at pull request page level. but we can design it at pull request level.
a rough thought:

( ( number of  finished open pull requests ) + ( number of  finished closed pull requests ) ) / ( ( number of  total open pull requests ) + ( number of  total closed pull requests ) )

( ( number of total open pull requests ) + ( number of total closed pull requests ) ) actually is number of repositories * 2