Implement Homepage badges for default theme

Add the following badges to the homepage.
- GitHub buttons
- Travis CI
- Coveralls

Example use case mkdocs.yml
```
github_buttons: ['tomchristie', 'flask-api', 'watch', 'true']
travis_ci: ['tomchristie', 'flask-api', 'master']
coveralls: ['tomchristie', 'flask-api', 'master']
```