Make the appending of ${sourceSet.name} to ${generatedSourceDir} optional

In my case I have the directory layout:

    src/main/java
    src/test/java

And I actually want my generated sources to end up inside src/main/java, alongside my normal sources. Currently it's impossible to turn off the appending of ${sourceSet.name} to ${generatedSourceDir}, so with

    generatedSourceDir = "src/main/java"

my sources would end up in "src/main/java/main" (!).

This pull request adds the appendSourceSetName boolean property for controlling the appending of the current ${sourceSet.name} to ${generatedSourceDir}. The default value is true (it will be appended).