closes issue #7: decoupling the registration of the groups from the configuration of the groups

Goals:

 * registration and setting for a group can happen before of after the other without any impact in the overall output. 
 * introducing `app.set('yui default base', 'http://path/to/cdn/{{groupDir}}/');`
 * introducing `app.set('yui default root', 'something/{{groupDir}}/')'`
 * introducing `app.set('yui combo config', { comboBase: '/combo?', comboSep: '?', maxURLLength: 1024})`
 * removing `app.yui.setGroupFromCDN()`, use `setGroupConfig` instead.
 * removing `app.yui.setGroupFromAppOrigin()`, that's the new default behavior and you can use `setGroupConfig` if you need more granularity.
 * removing `app.yui.combineGroups()`, it will inherit from the top level `combine` value.
 * from now on, the folder that represents the build directory for the bundle will be used as the `root` for loader, so the version of the bundle will be included.
 * `combine` is now inherited by default from top level or defaults to `true`
 * `filter` is now inherited by default from top level or defaults to `min`

As part of the PR, we are also automatically setting any group as app origin at the moment they are registered without having an explicit configuration upfront.