Introduce new xwiki-classloader module

To offer common apis when manipulating classloaders.

Initial features:

• Fixes several of the JDK's pb with the URLClassloader, see http://dcl.mathcs.emory.edu/php/loadPage.php?content=util/features.html#classloading
• Introduces a default URL Stream Handler that locates URL Stream Handlers as components
• Rewrite a JAR URL Stream handler so that jar urls can support custom protocols even if they are no globally registered (see http://accu.org/index.php/journals/1434 for more details)
• Write an URL Stream handler that knows how to handle JARs attached to wiki pages