fix API (session, sid)

Minor changes in 'Protocol_API.js':
* change __'session=Synoloader'__ to __'session=DownloadStation'__
* change all __'sid='__ to **'_sid='**

According to Synology Download station API 20140326 this is how they should be. I'm very new to Synology API so I don't know did they change those param definitions.

With those changes my issue with connection is solved