Gradle build

I removed IntelliJ-related files and some other unnecessary files.
Then I create a basic Gradle build file.
The script should automatically download all the required dependencies.

From IntelliJ IDEA you can open the project simply selecting the build.gradle file.
I'm still getting compile errors in ToggleSwitch class, it seems the `javafx.scene.accessibility` package doesn't exist in JDK 8.05, but I haven't spent much time on it.