Move Fusion support into a separate module

[mcollovati]

vertx-vaadin has its own implementation of Fusion endpoints, to avoid Spring dependencies; some components has been completely rewritten to abstract over servlet and spring stuff, reusing original classes (with some workaround) when possibile.

Since Fusion is packaged separately from Vaadin server, it may be better introduce the same separation also on vertx-vaadin, to avoid adding useless dependency when not needed.

New module may be named vertx-fusion-endpoint, to be consistent with Vaaadin fusion-endpoint module