Fix CORS for st2auth

STORM-862
Replace Auth middleware with hook

This is needed for CORS headers to apply to authentication errors too. Hook is handled after the middleware so request may never reach it.
Handle OPTIONS request correctly
Return a better and more friendly error message if a pack virtualenv doesn't
exist.
Include exception info.
Add version badge

the current version is taken from 'release' variable,
but the previous versions are hardcoded, need to be
updated manually every big release for now.
Merge pull request #838 from StackStorm/DOCS/add_version_badge

Add version badge to Docs
Merge pull request #833 from StackStorm/feature/cors

Fix authentication to support CORS
Merge pull request #835 from StackStorm/virtualenv_error_message_improvements

Virtualenv error message improvements
Merge branch 'master' into services_version_reporting
Display a static page with a version and documentation URL instead when hitting
/.
Use st2common.__version__ attribute in the docs to determine the latest version.
Update github_repo attribute.