[WIP] adding webdav resource for activities

@PVince81 @evert please have a look

@dragotin this would be the resource to request activities from the server - as requested by @msrex 

url to query:
http://owncloud/remote.php/activities

![bildschirmfoto von 2014-12-17 23 39 31](https://cloud.githubusercontent.com/assets/1005065/5480386/08c59698-8646-11e4-8430-0ebd2636e531.png)
![bildschirmfoto von 2014-12-17 23 40 24](https://cloud.githubusercontent.com/assets/1005065/5480391/1b5f9b78-8646-11e4-8cda-9e080aeb8335.png)

Can we put this file in the app framework some how and only pass in `new \OCA\Activity\Sabre\Collection()` from the app?
Looks like it can be easily deduplicated.