Add map to event page and weekly invitations
https://github.com/railsgirlslondon/railsgirls-london/issues/157

To make it work on Heroku:
heroku config:set GOOGLE_MAPS_KEY=XXXXXXXXX
Get your developer key from:
https://developers.google.com/maps/documentation/javascript/tutorial#api_key
Merge pull request #184 from camilleldn/master

Add map to event page and weekly invitations