To make it easy to switch between running agents in the AgentServer,
i.e. threaded, and running them in their own process (in case of unstable
agents), we'll use the agent_launcher helper program soon. This will do
practically the same as the AgentServer, but only running one Agent.
This approach prevents AgentWriters to have to provide a plugin *and* an
executable.

svn path=/branches/work/akonadi-agentserver/; revision=1181544