A better fix for #113

I believe that the following is more semantically correct fix for #113. It builds upon what @aalmiray has already done. 

It breaks current form of property assignment in the plugin to get closer to Gradle convention
```groovy
sourceDocumentNames 'one.adoc', 'two.adoc'
sourceDocumentNames files('three.adoc','four.adoc')
sourceDocumentNames new File(projectDir,'src/asciidoc/four.adoc')
sourceDocumentNames {'five.adoc'}
```
In line with convention, now doing
```groovy
setSourceDocumentNames 'six.adoc'
```
will remove previous documents. I don't really see much of a use-case at present for this, so maybe we should throw this method out completely. It does however match behaviour with other gradle plugins

This does leave us with a dilemma that from a user perspective it is different to all of the other _calls_ in the configuration closure, but I actually think we should change most of them as well as some point. (I can raise a seperate issue for this).