- added an example for the stats component

- added terms query including unittests and an example
- added customize request plugig with unittests and example

- added an example script
- added unittests

Conflicts:
	library/Solarium/Client/Request.php

- small fixes and improvements to loadbalancer plugin

- added support for optional events to plugin system

- implemented failover in loadbalancer plugin
- added exclude option to weightedRandomChoice class

- refactored '$_plugins' to '$_pluginInstances'
- added autoCreate option to getPlugin
- added triggerEvent method for 'external' events

Made Zend HTTP Adapter use headers from request