Add single_version field to projects.models.
Add south migration for single_version field.
Handle single_version docs for '/docs/' URLs.

* Add SingleVersionMiddleware(), which handles '/docs/<project>/...' URLs.
* Add serve_single_version_docs() view function.
* Add new urlconf (single_version_urls.py)
* Doesn't yet handle subdomain URLs.
Add tests for SingleVersionMiddleware.

* Test '/docs/<project>' URLs only.
* Add "catchall" URL pattern so that CommonMiddleware doesn't
  append slash to valid single_version URLs like '/docs/pip/usage.html'.
* Revise tests that 404 directly now, rather than redirect and
  then 404.
Handle single_version docs for subdomains & CNAMEs.
Refactor SingleVersionMiddleware.
Add logging to SingleVersionMiddleware.
Don't blow up if single_version field is missing.
Merge pull request #591 from toffer/issue-293-single-version

Issue 293 single version