Add API for services to send log messages that can be displayed by the web client

[Nicklas Nordborg]

Services are extensions that are running in the background to perform some kind of regular service. They can be listed and accessed by the GUI at Administrate -> Services where it is possible to start and stop the services.

It would be useful (mostly for debugging purposes) to have a simple logging API that the services can use. It should be possible to view messages that are submitted to log in the web interface. The log doesn't have to be permanent but could be kept in memory with a limit on the number of messages to keep before older ones are removed.