OCS API for server-to-server sharing

OCS API for server2server sharing as part of #12285 .

This is work in progress, I just want to give people a chance to look at it early.

This is how the API will look like:

POST to create new server2server share
```
http://localhost/oc/ocs/v1.php/cloud/shares
```

post data: shareWith, token, name, remote, remote_id, owner

POST to notify that the share was accepted
```
http://localhost/oc/ocs/v1.php/cloud/shares/{shareId}/accept
```

post data: token

POST to notify that the share was declined
```
http://localhost/oc/ocs/v1.php/cloud/shares/{shareId}/decline
```

post data: token

POST to unshare server-to-server share
```
http://localhost/oc/ocs/v1.php/cloud/shares/{shareId}/unshare
```
post data: 'token'
Or as a practical example:

1. Login as user "admin"
2. Create a folder "foo"
3. Share "foo" publicy (`http://localhost/core/index.php/s/aGCjLi78JOzFua4`)
4. Delete all cookies
5. Send a decline request

```
➜  ~  curl -X POST http://localhost/core/ocs/v1.php/cloud/shares/1/decline -d token=aGCjLi78JOzFua4 -v
* Hostname was NOT found in DNS cache
*   Trying ::1...
* Connected to localhost (::1) port 80 (#0)
> POST /core/ocs/v1.php/cloud/shares/1/decline HTTP/1.1
> User-Agent: curl/7.37.1
> Host: localhost
> Accept: */*
> Content-Length: 21
> Content-Type: application/x-www-form-urlencoded
> 
* upload completely sent off: 21 out of 21 bytes
< HTTP/1.1 200 OK
< Date: Mon, 01 Dec 2014 18:18:10 GMT
* Server Apache/2.4.9 (Unix) PHP/5.6.2 is not blacklisted
< Server: Apache/2.4.9 (Unix) PHP/5.6.2
< X-Powered-By: PHP/5.6.2
< Set-Cookie: ocgns13bl46f=9b6krapplmghdc8n7fqfmndg06; path=/core; HttpOnly
< Expires: Thu, 19 Nov 1981 08:52:00 GMT
< Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
< Pragma: no-cache
< X-XSS-Protection: 1; mode=block
< X-Content-Type-Options: nosniff
< X-Frame-Options: Sameorigin
< Content-Security-Policy: default-src 'self'; script-src 'self' 'unsafe-eval'; style-src 'self' 'unsafe-inline'; frame-src *; img-src *; font-src 'self' data:; media-src *; connect-src *
< X-Robots-Tag: none
< Transfer-Encoding: chunked
< Content-Type: text/xml; charset=UTF-8
< 
<?xml version="1.0"?>
<ocs>
 <meta>
  <status>ok</status>
  <statuscode>100</statuscode>
  <message/>
 </meta>
 <data/>
</ocs>
* Connection #0 to host localhost left intact
```

6. Copy the cookie's value (`9b6krapplmghdc8n7fqfmndg06`) into your browser and have fun being logged-in.