base.html and HTML5

- Change layout.html to base.html - fixes #275
- Make layout.html just extend base.html
- Change templates to extend base.html
- Add doc that layout.html is deprecated
- Change base.html to HTML5 - fixes #270
Branch: feature/275-base
Merge branch 'release-0.12.0' into feature/275-base

Branch: feature/275-base
Merge pull request #275 from rapidsms/feature/275-base

Move to base.html and switch to HTML5
Fixes #275 and #270