refactor for loader patches

• add api to allow patching the YUI instance before the loader use statement
  • app.yui.patchServer(patchFn) for patching the server instance
  • app.yui.patchClient(patchFn) for patching the client instance
  • app.yui.patch(patchFn, anotherFn) for patching both the client and server instances
  • all default patches are accesible thru: require('express-yui/lib/patches/optional-requires')
• add default logLevel to be warn by default to avoid
  the masive amount of logs from yui core modules.

Important (non-backward compatible) changes:

• [!] Patches for loader to support templates: [] and Y.Template.get() are disabled by default.
• [!] Patches for loader to support langBundles: [] and Y.Intl.get() are disabled by default.
• [!] Patches for loader to support optionalRequires: [] is disabled by default.