adds eventbrite integration if venue.url matches eventbrite

We may want to abstract the logic further if we add facebook integration or other providers.  but considering a few sites used eventbrite last time I figured it was worth doing.

If your url includes eventbrite.com and an "eid" param, then it'll satisfy @venue.has_eventbrite

Also it's worth noting that it points at my toronto event id for the custom button, but until we invest more in this, i prefer it to the message "buy tickets".

![register_button](https://f.cloud.github.com/assets/711967/658578/d9e9b034-d5f2-11e2-9194-23075c36ccf6.png)
![ticket_embed](https://f.cloud.github.com/assets/711967/658579/d9faf434-d5f2-11e2-882d-842fb3880316.png)