Added VaadinApplicationContext including bean autoconfiguration

Added VaadinApplicationContext
Added VaadinApplicationContext as bean to autoconfiguration.

VaadinApplicationContext Allows to use the Spring application context as static from anywhere within the application.

Example usage:
EventBus eventBus = VaadinApplicationContext.getEventBus();

Reason for adding to vaadin-spring-boot:
Allows access to eventbus and applicationContext from NON-Managed Spring Beans.

Within the future VaadinApplicationContext can be extended with more helpful methods.