Introduce component architecture based on Plexus

Introduce the notion of components inside XWiki's core using Plexus. Namely:

• XWiki component lifecycle phases
• Plexus integration
• Servlet/Container component to bridge the gap between containers and XWiki internal objects so that xwiki can be used in different environments

See http://www.nabble.com/-XWiki-V2--Componentizing-XWiki-with-Plexus-tf4226730.html#a12024444