Setup a multi-project Gradle build

Changes include:
 - move all source to subprojects/testfx-core
 - apply consistent license header to all sources
 - enable code-quality and code coverage plugins
 - setup JavaFX jar lookup whether it's JDK7 or JDK8

TODO:
 - figure out why `publish` does not produce a pom file
 - setup publications via bintray maybe (?)
 - setup CI (Travis?)
 - setup project coverage via coveralls