fix API (session, sid)

Sorry for second pull request - It is my first time:)

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