Add PluginPermissionChanger module for managing per site plugin permissions

This is basically a thin wrapper around the firefox nsIPermissionManager service.
It provides simple access to set the status of the plugin and a callback mechanism
that notifies you of changed permissions. The therefore necessary observers are
managed automatically and removed on unload of the module.
You have to create an instance of the class per site and plugin.
Add option to disable flash on youtube (not enabled by default)

Some users want to use the youtube HTML5 player but still keep flash enabled on
all other sites. This uses the newly added PluginPermissionChanger.
Properly implement UserAgentChanger with automatic unloading

The UserAgentChanger is completely rewritten using js prototyping while almost
maintaining the interface; only the constructor parameters were changed.
Additionally an automatic unregistering of the observers on unload was implemented.