Added placeholders in action messages.
Added more platform-independent methods:
* added executeCommand to TabooPlayer and impl.
* added Layer interface and its implementation for methods that are
server-specific (rather than player-specific).
Improved built-in actions:
* added Command action
* added "to" parameter to Message action
* added a check for null in Action.format()
Added synchronization in TabooPlugin. Added missing space in some log
messages.