Method to add default facets because manually adding facets removes default facets

Hi @radimk, 

thanks to your comment on http://forums.gradle.org/gradle/topics/additional_eclipse_wtp_facet_overwrites_default_facets. I have fixed the behavior. Please tell me if this satisfies your needs and feel free to change/enhance the changed documentation or give me same feedback such that I can do!

Cheers,
Andreas