Make Multiplayer intentional instead of a hackjob. Also fix bugs.

[mgovea]

• bugfix: makeRidesFree button no longer affects non-rides (bug introduced in Update ride prices via Game Action, hopefully adding MP support #111, so things like shops were made free)
• bugfix: running the plugin on a server no longer complains about ui not existing (ReferenceError: identifier 'ui' undefined #113)
• bugfix: fix price calculation for parks which have previously had paid entry, had a non-zero price, then the flags were changed to have free entry (like my test park)
• feature change: only trigger 1x per day on server
  • aka no longer triggered automatically by clients
  • this also means for server play, the plugin MUST be installed on the server
• update README to acknowledge the compatibility with multiplayer