Use meetup.com for ATX meetup

The meetup.com page seems to be kept up to date more than the website.
Merge pull request #910 from owilliams/patch-2

Use meetup.com for ATX meetup
Grammar fix in routes guide.
Merge pull request #920 from steveklabnik/patch-1

Grammar fix in routes guide.
Merge branch 'master' of https://github.com/emberjs/website