OCS API for reading activities

@DeepDiver1975 @MTRichards 

Available routes:
ocs/v1.php/apps/activity/api/v1 `/filter/$filter` `/page/$page` `?allowgrouping=false`

Sample output can be found here:
https://gist.github.com/nickvergessen/d89b51a08904aecd0a54

Format (json/xml) and logging in works just like with the Sharing API