Make the default BOOTSTRAP_BINDER_NAME configurable

[galves]

It is a bit of a pain having to explicitly define the binder name for each verticle in the json deployment config, when I imagine most users have the same binder for each verticle deployment?

Also it creates a confusing error when the binder is not set:

"Guice bootstrap binder class com.englishtown.vertx.guice.BootstrapBinder was not found. Are you missing injection bindings?"

This should at most result in a warning because you may intentionally be using just a parent injector.

Please can we add a setter for it the binder in the verticle factory then pass it to the loader.

Thanks

[dano]

FWIW, the issue with the confusing error is fixed by #13