Overview of changes

• The Requests::VERSION constant has been updated to reflect the actual version for the release. @jrfnl, #485
• Update the .gitattributes file to include fewer files in the distribution. @mbabker, #484
• Added a release checklist. @jrfnl, #483
• Various minor updates to the documentation and the website. @jrfnl, @schlessera, #477, #478, #479, #481, #482