Gradle rework and example project

[Jolanrensen]

Helps with #59 and #64

Provides two example projects, one which was used in the demo video.

Creates this new Gradle notation:

kodex { // : KodexExtension
    preprocess(kotlin.sourceSets["mySourceSet"]) {
        generateJar = true
        generateSourcesJar = true
        processors = listOf(
             ...
        )
        ...
    }
}

which will:

• Attaches KoDEx to the given [sourceSet].
• Creates a new SourceSet named "${sourceSetName}Kodex" by default ([KodexSourceSetTaskCreator.newSourceSetName]).
• Creates a new task for the given source set named "preprocess${newSourceSetName}" by default ([KodexSourceSetTaskCreator.taskName]).
• This task will preprocess the given source set and store the results at
  build/kodex/$newSourceSetName by default ([KodexSourceSetTaskCreator.target]).
• Creates a jar task if [sourceSet] is the main source set, and we're not multiplatform
  ([KodexSourceSetTaskCreator.generateJar]).
• Creates a sources jar task if [sourceSet] is the main source set, and we're not multiplatform
  ([KodexSourceSetTaskCreator.generateSourcesJar]).

Refactored a lot on the gradle plugin side