Add plugin initialization at server start

Added the ability for a plugin to specify an initialization routine that
is only executed when the SiriProxy server is started. This provides the
plugin a mechanism to perform a task that is not required to be
performed upon every connection.
Simplify plugin init

Utilize the PluginManager to initialize routines instead of duplicating
code. Also globalized $LOG_LEVEL since it's required by PluginManager.
Merge pull request #453 from kylethedude/master

Allow plugins to initialize at server startup