Document building protocol-relative URLs

https://github.com/mitsuhiko/werkzeug/pull/528
Made FlaskGroup an AppGroup
AppGroups produce more AppGroups
Merge pull request #1158 from untitaker/protocol-relative

Document building protocol-relative URLs
Don't mention Flask dev releases anymore

Fix #1155
Merge pull request #1160 from untitaker/docs_dev_installation

Don't mention Flask dev releases anymore
Merge pull request #1161 from pbu88/master

Fix warnings and code styling in the doc