Add /activities and /activity endpoints

Hey @modemlooper,

I could be totally incorrect with my implementation of these endpoints but I believe based on the WP-API docs I think I'm on the right track with this PR using Basic Auth.

http://yoursite.com/wp-json/activities should show something like:
![get_activities](https://cloud.githubusercontent.com/assets/1377956/4582498/12a3ff12-4fe9-11e4-86b4-3fb453f6759e.png)

http://yoursite.com/wp-json/activity/id should show something like:
![get_activity](https://cloud.githubusercontent.com/assets/1377956/4582517/305dc7fe-4fe9-11e4-8f7b-a8e600ff099c.png)

If you can replicate this using Basic Auth and you're happy to merge it in then I will try and start adding more `WP_JSON_Server::READABLE` endpoints for all things BP next couple of weeks.

Once they are all in place we can then start tackling `WP_JSON_Server::CREATABLE`, `WP_JSON_Server::EDITABLE` and `WP_JSON_Server::DELETABLE` endpoints. It's gonna be a hell of a lot of work given how complex BuddyPress is but I'm pretty sure once we get the ball rolling yourself, @paulgibbs, @johnjamesjacoby and @boonebgorges might all want to chip away at this. I was working on a clients iOS web app a few weeks ago and wanted to extend the WP-API for BuddyPress but didn't have the time to write all the endpoints so I had to hack through it. I was using [Framework7](http://www.idangero.us/framework7/) just like @johnjamesjacoby and his BuddiOS project so I know he knows the pain of trying to working around a lack of BP-API! :+1: 

I'm not a great backend coder but I think I can get the ball rolling on this and seeing @rmccue normally works from my office I know he'll be quick to help out if we get stuck or need more filters :smile: