Always create eclipse wtp tasks if "eclipse-wtp" plugin is applied

As in the commit comments described I fixed GRADLE-2186 and GRADLE-2221 by doing this. Also I added a lot more tests and found an error within documentation. 

Furthermore please have a look at the documentation problem and my question reported in http://forums.gradle.org/gradle/topics/additional_eclipse_wtp_facet_overwrites_default_facets to provide another pull request :-)

And maybe you can also answer/give me a hint for my question in https://issues.gradle.org/browse/GRADLE-2362?focusedCommentId=19962&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-19962 to provide the next pull request :-)