Added issue and pr roles.

This commit also split news.rst and 2014-news.rst. news.rst will
only contains latest unreleased version's changelog.
Fix ref URL of the pr role.
Fix header in news.rst.
Fix typo WSGIErrorsWraper -> WSGIErrorsWrapper
Remove unused imports.
Simplify README.rst.
Document statsd_* commands.
Fix settings reference.
Fix history header in news.rst.
Add ref links to configure.rst and faq.rst.
Improve the "Running Gunicorn" command.
Fix syntax highlighting in configure.rst.
Resort docs index.
Simplfy documentation index page.
Add missing period.
Remove an incorrect note from instrumentation.txt.