Add Google Analytics code [JOINDIN-303]

Also made some small other changes, so that I could make this happen:
- Send the app environment (whether in dev or live mode) to the template using variable joindin_env without needing to send it from the controller (the main template doesn't have a controller)
- This also means we can change the way we check the environment in previous code, so I've done that
- Moved JS to the bottom of the page for quicker loading times (and as recommended by Google Analytics implementation advice), and renamed the codeblock so it remains semantic
Changes made in response to comments [JOINDIN-303]

- Rename environment variable to 'slim_mode'
- Switch around the logic for the Google Analytics, so it displays when the mode is live rather than "not development'
- Put the Google Analytics ID into the config file
Merge pull request #33 from clairs/JOINDIN-303_analytics_js

Add Google Analytics code [JOINDIN-303]