Github integration to attend Meetups

I integrated Github using their OAuth API. The application flow is as follows:

* Link: "Sign in using Github"
  * on click: OAuth token exchange, create `Member`, create session for `Member`
* Link: "I will be there!"
  * needs session
  * creates a `Participation`
* Link "I can't make it"
  * needs a session
  * needs a participation
  * destroys the `Participation`

Currently all the Links are displayed in the meetup metadata but this can change of course:

![Screen Shot 2013-03-02 at 12 39 08 PM](https://f.cloud.github.com/assets/5402/212761/dcc05014-832d-11e2-8805-c2b881b4d660.png)

## Design Work to do:

* We need styling for the three links
* We need styling for the list of attendees

## Deployment Work:

In order to make the github integration work we need to set two environment variables:

* GITHUB_SECRET
* GITHUB_CALLBACK = "http://ruvetia.org/auth/github/create"