License meetup data under CC-BY-4

Opening this PR mainly for discussion. In https://github.com/knode/meetups/pull/14 @chrisdickinson wrote:
> We should consider putting the rest of the information under a permissive creative commons license.

The CC-BY license is similar in spirit to the MIT license: it requires attribution, but otherwise places no restrictions on downstream use, modification, and redistribution of the content. The versions in CC licenses reflect their legal text as it is refined for use in as many jurisdictions as possible- afaik the intent of the various CC licenses has not changed since their inception.

I added a note in LICENSE.md about user contributions being licensed under this CC license. StackOverflow [addresses this in their terms of service](http://stackexchange.com/legal/terms-of-service#3SubscriberContent).

Is it worth licensing data separate from code?

Also, and this might be for a separate PR, but I like the idea of explicitly assigning copyright ownership to the respective contributors.