OCS Share API

This pull request implements a OCS API for file sharing. The base URL is ocs/v1.php/apps/files_sharing/api/v1/

Please test and review the already implemented API. Before the final merge I will at least add the "send mail notifications to users for non-public shares" feature, described below.

Following calls are implemented (changed 17.Sep., I now switched to the alternative API discussed below):

- /shares 
  - GET get all shares (/shares?file=/foo.txt would give us the shares for a given file/folder; /shares?file=foo&subfiles=yes will return all shares from all files in the folder specified in 'files=') 
  - POST create new share

- /share/shareID
  - GET get information about the specific share
  - PUT updates the specific share
  - DELET delete the specific share

Missing:

- send mail notification to users for non-public shares, waiting for https://github.com/owncloud/core/pull/4689. Will implement it as optional parameter for the share operation, not sure if we need it also as separate call to trigger it after the file was share, opinions?

- send public link as mail. I would like to hear some opinions, do we need this as API call for the clients or should we utilize the clients instead. Especially for the mobile clients we could use local apps like email client, twitter client, sms,... to share a generated link instead of telling the ownCloud server to send a mail.