Added webResourcesValidators extension point. fixes #14

This fixes #14 

Take a look at the adopter side of this extension point here: https://github.com/gamerson/liferay-ide/commit/b6503ad4e50b015c0f0d4b16b123a4246d914da1
With IWebResourcesValidator  name, you expect to have some validation, although it is used just to ignore validation. If tell me if we have other features for validation, we could create an new interface API. That's why I think IWebResourcesIgnoreValidator  is better. This API follows the same idea than the future WTP ignore element/attribute validator https://issues.jboss.org/browse/JBIDE-18387